n o t e s grade: 12 subject: biology topic: inheritance … · bio - notes - inheritance page 4 of...

BIO - Notes - Inheritance of 24

N O T E S GRADE: 12 ___ SUBJECT: BIOLOGY TOPIC: INHERITANCE DATE: ____________

STUDENT’S NAME: _____________________________________________________


Inheritance


▪ Inheritance or heredity is the transmission of genetic information from one generation to the

next, leading to continuity of the species and variation within it. ▪ The study of inheritance is called Genetics. ▪ A most conspicuous structure in all eukaryotic cells is a nucleus that controls the cell division. ▪ The nucleus contains thread like structures called chromosomes. ▪ Chromosomes are the vehicles of inheritance. ▪ Inheritance is defined as the transmission of genetic information from generation to generation. 3.1 Chromosomes Define the terms: ▪ Chromosome as a thread of DNA, made up of a string of genes ▪ Gene as a length of DNA that is the unit of heredity and codes for a specific protein. A gene may

be copied and passed on to the next generation ▪ Allele as any of two or more alternative forms of a gene ▪ Haploid nucleus as a nucleus containing a single set of unpaired chromosomes (e.g. sperm and

egg) ▪ Diploid nucleus as a nucleus containing two sets of chromosomes (e.g. in body cells) Chromosomes:

▪ A chromosome is a long, fine thread-like structure of DNA and protein (histones) made up of a string of genes.

▪ In a non-dividing cell they appear as a chromatin network while during cell division they become condensed to form short and thick chromosomes.

▪ They can be seen in the cell by light microscope only when the cell is dividing because at this time they become shorter and thicker.

▪ Chromosomes exist in pairs and each chromosome is made up of two strands - each strand is called a chromatid.

▪ The number of chromosomes is characteristic of the species. (E.g. human cell contain 46 chromosomes, fruit fly-8 etc.)


Types of chromosomes:

1. Autosomes. 2. Sex chromosomes

Autosomes: The chromosomes which are responsible for different characteristics except sex. Sex chromosomes: The chromosomes that is responsible for sex determination. They are represented as X and Y. Y chromosome is shorter than X. Homologous chromosomes: Chromosomes which carry genes for the same characteristics in the same position, having same

length, thickness and number of genes are termed as homologous chromosomes. Genes: A length of DNA in a chromosome which codes for the formation of a specific protein controlling a

specific characteristic of the organism. They are the units of inheritance. Alleles: Alleles are pairs of genes that are responsible for a pair of contrasting characters e.g. tallness and

shortness in height. They are alternate forms of the same gene.


Diploid nucleus: A nucleus containing two sets of chromosomes. E.g. in somatic (body) cells. In humans the diploid number is 46. Haploid nucleus: A nucleus containing a single set of unpaired chromosomes. E.g. in sperm and ova (eggs). In humans the haploid number is 23. Determination of sex: Describe the inheritance of sex in humans (XX and XY chromosomes). The sex of a child depends on one particular pair of chromosomes called the sex chromosomes. X – Represents the Female genotype Y – Represents the Male genotype In females, the two sex chromosomes, called the X chromosomes, are of the same size and so they

have genotype XX. In males, the two sex chromosomes are of different sizes. One corresponds to the female sex

chromosomes and is called the X chromosome. The other is smaller and is called Y chromosome and so their genotype is XY.

Analysis:


MITOSIS ▪ Define mitosis as nuclear division giving rise to genetically identical cells in which the

chromosome number is maintained by the exact duplication of chromosomes (details of stages

are not required).

▪ State the role of mitosis in growth, repair of damaged tissues, replacement of worn out cells and

asexual reproduction.

1.2.1 Mitosis: (Equational division):

• A type of cell division in which the new cells are genetically identical to the original, is known as

mitosis. During the process, all the chromosomes are copied (duplicate) and split to form two

nuclei with same number of chromosomes as the parent nucleus cell (diploid number of

chromosomes are maintained).

Identify the stages of mitosis: It takes place in somatic cells. 1. In prophase, the chromosomes become visible with two chromatids.

2. In metaphase, the chromosomes line up across the middle of the cell.

3. In anaphase, the chromatids separate and move to opposite ends of the cell and the cell starts to

split into two.

4. In telophase, the chromatids become chromosomes and the cell now completely splits into two.


Significance of mitosis: Mitosis is important for three reasons: 1.Growth:

The number of cells within an organism increases by mitosis and this is the basis of growth in multicellular organisms.

2.Repair: Cells are constantly dying and being replaced by new ones in the skin and digestive tract. When damaged tissues are repaired, the new cells must be exact copies of the cells being replaced so as to retain normal function of cells.

3. Asexual reproduction: Mitosis is used as a form of asexual reproduction in some organisms like in unicellular Amoeba and

multicellular hydra as well as vegetative reproduction in plants (i.e. some plants produce offspring which are genetically similar to themselves. These offspring are called clones)

Some animals can regenerate parts of the body and production of new cells is achieved by

mitosis. NB: Cells or organisms that are genetically identical are known as clones. MEIOSIS

• Define meiosis as reduction division in which the chromosome number is halved from diploid

to haploid (details of stages are not required)

• State that gametes are the result of meiosis

• State that meiosis results in genetic variation so the cells produced are not all genetically

identical.

Meiosis: a reduction division in which the chromosome number is halved from diploid to haploid.

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Meiosis involves two sequential cycles of nuclear and cell division called: 1) Meiosis I: Reduction division 2) Meiosis II: Like mitosis; Equational division

• Meiosis takes place in reproductive organs.

• Occurs during sexual reproduction to produce gametes (sperm/pollen cell and egg cell)

• At the end, four daughter cells are produced.

Note: The offspring produced by sexual reproduction are genetically different from their parents

because of a process called “crossing over”. The cross-over occurs in the prophase of meiosis I. The reason is to increase the diversity of the

offspring and make them more genetically distinct. Here the chromatids from the two sister chromosomes will break off a piece of the cromatid and ‘swop’ with the other chromosome. This is to ensure that no genetic a information is lost and that genetic information is always a combination of both parents. (This is why you will have features of both your mother and father).


GENETIC CONTROL AND PROTEIN SYNTHESIS Almost all cells contain two types of nucleic acids: 1. DNA (Deaxyribo-Nucleic Acid) 2. RNA (Ribo-Nucleic Acid) The structure of nucleic acids: Both DNA and RNA are nucleic acids, polymers / macromolecules,

made up of many smaller molecules / monomers called nucleotides. The DNA and RNA are polynucleotides. . The nucleotide is made up of 3 smaller molecules:

1. A Phosphate group 2. A Pentose Sugar (either ribose or deoxyribose) 3. A Nitrogen base

There are five types of Nitrogen bases, which belong to one of the two groups: P = Phosphate group S = Sugar N = Nitrogen base Each nucleic acid contains four types of bases only in its structure. The DNA molecule contains, the

bases A, C, T, G while the RNA contains A, C, U, G The structure of DNA molecules: proposed by Watson and Crick. ▪ It is made of two polynucleotide strands lying side by side. The two strands are held together by H

bonds between the nitrogen bases according to the rule of base pairing A = T, C = G. ▪ The two strands twist around each other to form a double helix, they run in opposite directions /

anti-parallel to allow the bases to fit exactly with each other. ▪ It is clear that in each base pair, one base belongs to the double ringed purines, while the other

belongs to the single ringed pyrimidine. ▪ The A = T pair has two H bonds, which G = C pair has three H bonds. The base pairing helps to:

▪ Stabilize the molecules ▪ Allow the DNA to replicate ▪ Give the DNA the three-dimensional configuration ▪ The width of the DNA molecule is about 2 nm; each twist contains 10 pairs of nucleotides and

measures 3.4 nm in length. The structure of Chromosomes: The chromosomes of an eukaryotic cell are made of: 1. DNA 2. Proteins (mainly histones) The chromosome is a single DNA molecule that wounds up. The human cell contains 46

chromosomes i.e. 46 DNA molecules. The total length of the molecules is about 2 meters. The histone proteins form a precise architectural skeleton that helps in the packing of the DNA molecules.

In an actively growing cell, the chromosomes are present as long chains, however when the cell is preparing to divide the chromosomes coil up and condense.


The DNA carries the instructions for proteins synthesis; specify the sequence in which amino acids are linked together primary structure.

Differences between DNA and RNA

DNA RNA

Type of sugar Deoxyribose Ribose

Types of bases A, G, T, C A, G, U, C

Structure Double strands Single strands

Locations Nucleus / some organelles Cytoplasm / nucleus

Replication Can Cannot

Stability More stable Less stable

Existence Permanent Temporary

Ratio between bases A / T, G / C = 1 A / U, C / G varies

Amount Constant for all cells

(except gametes)

Varies from one cell to cell

according to metabolic

activities

DNA Replications: Before a cell divides, its DNA is replicated / duplicated (in the interphase), so that

each new cell receives a complete copy of the original genetic information. Mechanism of DNA replication: 1. The two strands unwind / unzip, due to the break down of H bonds between the bases by helicase

enzymes. 2. Each one of the two strands acts as a template, to which a complementary set of nucleotides

(always present in the nucleus) would attach according to the rule of base pairing by H bonds. 3. The sugar of one nucleotide is joined to the phosphate of the next nucleotide to form a new

polynucleotide chain. (An enzyme called DNA polymerase catalyzes the joining of nucleotides). 4. The replication process is called semi conservative because each new double helix has one old

strand (parental) strand and one new strand. In other words, one of the parental strands is conserved / present in each doughter double helix.

Requirements of Replication: 1. Free nucleotides 2. Enzymes (helicase, polymease) 3. Energy source (ATP) to activate the nucleotides. PROTEIN SYNTHESIS: ▪ Watson and Crick suggested that the genetic information which passed from one generation to

another, mighty reside in the sequence of bases of single DNA strand. Therefore the DNA nucleotide base sequence determines the amino acid sequence of protein molecules.

▪ The single DNA molecule contains shorter sections called genes. ▪ Gene is a sequence of nucleotides / parts of DNA molecules which codes for a polypeptide. ▪ The instructions are coded only in one of the two strands of the DNA molecules sense strands,

while the other strands (nonsense) tends to stabilize the molecule and allow it to replicate.


Triplet Genetic Code: Triplet code is the normal version of the genetic code in which a sequence of

three nucleotides codes for the synthesis of a specific amino acid A row of three bases called a codon specifies one amino acid i.e. the genetic code is said to be triplet. Why is the code triplet? 1. If one base determines the position of a single amino acid in a protein, therefore the protein could

only contain four amino acids (41) 2. If two bases code for amino acid, the 16 amino acids could be specified into the protein molecules

(42) 3. Only a code composed of three bases could incorporate all 20 amino acids into the structure of

protein molecule. Such a code would produce 64 combinations of bases (43). Ribonucleic acid (RNA) structure: Ribonucleic acid RNA is a polymer made up of repeating mononucleotide sub-units. It forms a single

strand in which the pentose sugar is always ribose and the organic bases are adenine, guanine, cytosine and uracil. There are three types of RNA, all are synthesized from DNA and involved in the protein synthesis.

1. Ribosomal RNA (rRNA) 2. Transfer RNA (tRNA) 3. Messenger RNA (mRNA) 1. Ribosomal RNA (rRNA): involved in the translation of mRNA into a sequence of amino acids in a

polypeptide chains at ribosomes. Ribosomal RNA (rRNA) is a large, complex molecule, which is a major component of ribosome’s, making up over half of their mass. It has a sequence of organic bases, which is very similar in all organisms.

2. Transfer RNA (tRNA): transfers amino acids from the amino acid pool in the cytoplasm to

ribosome. Each amino acid has its own RNA. Although there are a number of types of tRNA, they are very similar, each having a single stranded chain folded into a cloverleaf shape, with one end of the chain extending beyond the other. This extended chain always has the organic base sequence of cytosine-cytosine-adenine; this is the part of the tRNA molecule to which amino acids can easily attach. There are at least 20 types of tRNA each able to carry a different amino acid.

At the opposite end of the tRNA molecule is a sequence of three other organic bases, known as the anitcodon. For each amino acid there is a different sequence of organic bases on the anitcodon. During protein synthesis, this anticodon pairs with the complmentary three organic bases that make up the triplet of bases on mRNA, known as the codon. The tRNA structure with its end chain for attaching amino acids and its anticodon for pairing with the codon of the mRNA, is structurally suited to its role of lining up amino acids on the mRNA template during protein synthesis.

3. Messenger RNA (mRNA): carries the instructions for protein synthesis from the nucleus to the

ribosome. Consisting of thousands of mononucleotides, mRNA is a long strand, which is arranged, in a single helix. Because it is manufactured when DNA forms a mirror-copy of part of one of its two strands, there is a great variety of different types of mRNA. Once formed, mRNA leaves the nucleus via pores in the nuclear envelope and enters the cytoplasm. Where it associates with the ribosomes. There it acts as a template upon which proteins are built.


Transcription: ▪ It is the mechanism by which the base sequence of a DNA strand is converted into the

complementary base sequence of mRNA. ▪ A specific length of the DNA molecule, which codes for a polypeptide unwinds by the breakage of

H bonds between the bases of the complementary strands. ▪ One of the DNA strands (sense) acts as a template for the formation of complementary strands of

mRNA. This molecule is formed by the linking of free ribonucleotides under the influence of the RNA polymerase enzyme and according go the rule of base pairing except U instead of T

▪ The mRNA leaves the nucleus through pores to reach the ribosomes in the cytoplasm. Translation: It is the mechanism by which the triplet base sequence of mRNA is converted to a

specific sequence of amino acids in a polypeptide chain. It occurs at ribosomes: 1. The mRNA is attached to a ribosome at a site called ribosome-binding site. 2. The ribosome accommodates two tRNA molecules that is it contains two tRAN sites where a codon

from mRNA can attach by base pairing to a molecule of tRNA. Each tRNA has a special triplet of nucleotides called anti-codon at one end. At the opposite end, the tRNA is linked to a specific amino acid (amino acid binding site).

3. The 1st codon to be translated from a mRNA molecule is always the sequence AUG which corresponds to the amino acid methionine and is called initiation codon.

4. When the two amino acids are brought together, they are linked to each other by peptide bonds (by condensation reaction) which catalyzed by an enzyme). At the same time, the 1st tRNA is disconnected from its amino acid and leaves the ribosome, which moves along the mRNA to bring the next codon into position. As this process continues, the ribosome travels a along the mRNA strands adding more amino acids to the growing polypeptide chain.

5. The sequence of the ribosome reading and translating the mRNA codes continues until it comes to a codon-signaling stop (UAA / UAG / UGA)

Features of DNA molecule that allow it to act as a genetic material: ▪ Replicate: being copied perfectly so it can pass unchanged into the new cells that are produced

when an old cell divides. ▪ Store information: for making protein that determines the characteristics of the organism and

since it is a stable molecule, the information remains intact from one generation to the next. DNA controls all the metabolic reactions: ▪ Enzymes control all metabolic reactions. Enzymes are protein in mature and so their synthesis is

controlled by the DNA and so the DNA controls the metabolic reactions. ▪ Any change in the DNA that codes for one enzyme leads to a stop of a chemical reaction, which in

turn leads to metabolic disorder. What do we know?

• All cells except the gametes (reproductive cells) consists of Diploid number of Chromosomes example human cells 23 pairs.

• The Sperm Cells and Egg Cells consists of 23 Chromosomes. ----- Haploid Cell.

• The creation of the first cell is the sharing of the chromosomes (nucleus) between a male and a female cell.

• Zygote (46) = sperm cell (23) + Egg Cell (23).


There are two basic types of nuclear divisions: 1. Mitosis: which results in all daughter cells having the same number of chromosomes as the

parent. 2. Meiosis: which results in the daughter cells having only half the number of chromosomes

found in the parent cell. (Sex cell). Chromosomes

• In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes.

• Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure.

• A chromosome often refers to a long strand of DNA that has been wound round like a twisted piece of string.

• This happens so that the extra-long bit of DNA will fit into the nucleus of the cell.

• The word chromosome actually means coloured body. It was called this because some chemical dyes made the chromosomes stand out under the microscope.

• Many cells have more than one strand of DNA in their cells. These strands are wrapped up to form chromosomes.

• Different species have different numbers of chromosomes. The structure of chromosomes

• The chromosome is made up of two identical structures called chromatids. • This is because during nuclear division (Interphase) each DNA molecule in a nucleus makes

an identical copy of itself. • Each DNA is contained in a chromatid and the two chromatids are held together by a

characteristic narrow region called the centromere. • Chromatids X 2 + Centromere = Chromosome. • Each chromatid contains one DNA molecule. • A DNA is the entire sequence of genetic info. • A Gene is a single expression e.g. the gene for a

ligament. • It controls 1 characteristic of the organism. • The gene for a particular characteristic is always

found in the same position (locus/loci) on a chromosome.

• Each chromosome has typically several hundred to several thousand gene loci.

• Humans are thought to have about 30 000 genes. • The fact that the two DNA molecules in sister chromatids, and

hence their genes are identical is the key to precise nuclear division.


Condensed Chromosome

Mitosis

➢ When cell division occurs, the nucleus divides first. ➢ In mitosis, the chromosomes, present as the chromatids formed during interphase, are

separated and accurately and precisely distributed to two daughter nuclei. 4 Phases of mitosis:

1. Prophase ➢ is a stage of mitosis in which the chromatin condenses (it becomes shorter and fatter into a

highly ordered structure called a chromosome in which the chromatin becomes visible.

➢ In prophase, the chromatin condenses into discrete chromosomes. The nuclear envelope (membrane) breaks down and centrioles form at opposite "poles" of the cell.

➢ A centriole is a small set of microtubules arranged in a specific way. (scaffolding)

• By the end of prophase the nuclear

envelope has entirely vanished and the chromosomes have condensed (they are tightly coiled and easily visible under a light microscope)


2. Metaphase

➢ In which condensed & highly coiled chromosomes, carrying genetic information, align in the middle of the cell before being separated into each of the two daughter cells.

➢ It is the alignment of the chromosome in the middle of the cell.

➢ In Metaphase, the centromeres of the chromosomes convene themselves on the metaphase plate (or equatorial plate), an imaginary line that is equidistant from the two centriole poles.

3. Anaphase

➢ Now, during anaphase, the two sister chromatids of each chromosome are pulled apart by the spindle and dragged by the spindle fibres toward opposite poles of the cell (i.e., toward the opposite centrioles).

➢ The movement results from a shortening of the spindle microtubules. Each chromosome is pulled along by its centromere.

➢ At the end of anaphase, each pole contains a complete compilation of chromosomes.

4. Telophase

➢ Nucleolus and nuclear envelope re-forms ➢ Chromatids have reached the poles of the spindle, they

will now un-coil again ➢ Centrosome will replicate before next nuclear division

Cytokinesis

➢ Cell organelles such as mitochondria and chloroplasts become distributed evenly between the cells.

➢ In animal cell division occurs by in-tucking of the plasma membrane at the equators, pinching the cytoplasm in half.

➢ In plant cells, the Golgi apparatus forms vesicles of new cell wall materials which collect along the line of the equator of the spindle known as the cell plate.

Meiosis

• Meiosis allows the child to have the genes of both the parents, without meiosis further generations would only be copies of either one of the parents which means that no new organisms will be formed and we would only have copies. This means that there would be no variety.


Significance of Mitosis and Meiosis Mitosis and meiosis both involve cells dividing to make new cells. Mitosis division can be for growth,

maturity, or to repair/replace damaged cells. Meiosis is basically required for reproduction to take place. Meiosis produces gametes, which are required for sexual reproduction. Thus, mitosis is important for the individual's health and wellbeing.

Monohybrid inheritance: Monohybrid cross: A monohybrid cross is a cross between organisms which show contrasting variations of only one

characteristic. Genotype: The genetic make - up of an organism, e.g. Tt, where T and t are alleles of a gene. Phenotype: The characteristics visible in an organism, controlled by the genotype, e.g. a tall plant or dwarf plant.

This is what you physically can see. Homozygous: (Pure)

• Having a pair of identical alleles controlling the same characteristics, e.g. TT or tt . Two

identical homozygous individuals that breed together will be pure-breeding.

Heterozygous: (Hybrid) Having a pair of dissimilar alleles for a characteristic, e.g. Tt. Not pure breeding. Dominant: An allele that is expressed when it is present. (E.g. TT) In other words an allele that always shows in the phenotype of an organism whether the organism is

homozygous or heterozygous.


Recessive: An allele that is only expressed when there is no dominant allele of the gene present. It means it can express itself in the phenotype only in a homozygous organism i.e.(tt) Note: The dominant gene is represented by a capital letter. The recessive gene is represented by the

small letter. Mendel’s Experiments: 1. A cross between a pure - breeding tall pea plant and a pure - breeding dwarf pea plant. i.e. Two

homozygous individuals with different phenotype:

2. A cross between two heterozygous tall pea plants from the offspring.i.e.Two heterozygous

(hybrids) individuals with the same phenotype:

Parents Male Female

Phenotype Tall × Dwarf

Genotype TT × tt

Gametes T T × t t

Punnett square T T

t Tt Tt

t Tt Tt

F1 genotype (first filial generation) all Tt F1 phenotype all tall The result is 100% heterozygous dominant.

Parents Male Female Phenotype Tall × Tall

Genotype Tt × Tt

Gametes T t × T t

Punnett square T t

T TT Tt

t Tt tt

F2 genotype (2nd filial generation) 1 TT, 2 Tt, 1 tt F2 phenotype tall, tall, dwarf The result is: The genotype ratio is: 1 homozygous dominant : 2 heterozygous dominant : 1homozygous recessive The phenotype ratio is: 3 dominant : 1recessive or 75% : 25%


Test cross: (or back cross) It is a cross used to find whether the dominant parent is homozygous or heterozygous for its

genotype. It can be obtained by making a cross between a heterozygous tall pea plant and a dwarf pea plant.

i.e. Heterozygous dominant and homozygous recessive:

More examples on Monohybrid cross: 1. The ability to roll the tongue is due to a dominant gene, which we will call R. The alternate

recessive gene for non-tongue rolling is r

What will be the outcome if? (a) a homozygous tongue roller mates with a non-tongue roller (b) a heterozygous tongue roller mates with another heterozygous tongue roller (c) a heterozygous tongue roller mates with a non-tongue roller Solution:

Parents Male Female Phenotype Tall × Dwarf Genotype Tt × tt

Gametes T t × t t

Punnett square T t

t Tt tt

t Tt tt

The phenotype ratio is: 1 dominant: 1 recessive or 50%: 50% The genotype ratio is: 1 heterozygous dominant: 1 homozygous recessive

(a) Phenotype of parents (p): roller non - roller Genotype of parents (p): RR rr

Gametes: R R r r

First generation(F1) Rr Rr Rr Rr

Genotypes: all Rr (100% Rr) Phenotypes: all rollers (100% rollers)


2. Mendel crossed pea plants having round seeds among themselves (self pollination). In a sample

of 39 seeds, he obtained 29 seeds which were round and 10 seeds were wrinkled.

a) Use a genetic diagram to explain the heredity.


3. A man with myopic eye married a normal woman. They got a normal child. Use a genetic diagram

to explain the heredity, knowing that myopic eye dominates normal eye. Parents phenotype: Genotype: Gametes: Offspring genotype: Offspring phenotype: Ratio:

b) Which is the dominant gene? Give reason:

…………………………………………………………………………………………………………………………

…………………………………………………………………………………………………………………………

…………………………………………………………………………………………………………………………

…………………………………………………………………………………………………………………………

…………………………………………………………………………………………………………………………

Phenotype of parents red flower x white flower

Genotype of parents CRCR x CWCW

Gametes CR CR x CW CW CR CR Punnett square CW CRCW CR CW CW CRCW CR CW F1 genotype All CRCW

F1 phenotype All pink


Co-dominance: The term describes a pair of alleles, neither of which is dominant over the other. This means that both the alleles express themselves in the phenotype, no matter what the

combination is going to be. In co-dominance, while writing the genotype of co-dominant alleles, the capital letter is used to

represent the gene involved for the character and a small raised (superscript) letter for each phenotype.

Example: A cross between two similar plants having red flowers and white flowers. Inheritance of A, B, AB and O blood groups: Blood group is an example of co-dominance between blood groups A and blood groups B as well as

dominancy because blood group O is recessive. It is controlled by three alleles IA, IB and IO. IA, IB (dominant) IO (recessive) Combination of three alleles results in four different blood groups. Example:

Two parents have blood groups A and B. The father is IA IO and the mother is IB IO.

Blood groups

Genotype Can give blood to Can receive blood from Homozygous Heterozygous

A IA IA IA IO A, AB A and O

B IB IB IB IO B,AB B and O

AB - IA IB AB only A, B, AB and O (Universal -recipient)

O IO IO - A, B, AB, O Only O (universal donor)

The alleles are responsible for producing antigens that respond to foreign antibodies. This can result in blood clotting in blood transfusion and rejection of organs after transplant operations.

Sex linked inheritance: Some genetic disorders such as red – green colour blindness and hemophilia are more common in

men than in women. This is because the recessive allele causing the disorder is found on part of the X chromosome which is not found on Y chromosome since Y chromosome is smaller.

Phenotype of parents blood group A blood group B

Genotype of parents IA IO IB IO

Gametes IA IO IB IO

F1 genotype IA IB IA IO IB IO IO IO

F1 phenotype AB A B O

Ratio 1 : 1 : 1 : 1


It means that males only need one allele to show the disorder while females need two copies. The pedigree or family tree showing sex linked inheritance. In family tree, Represents a male. Represents a female. A horizontal line joining them means a cross between them. A vertical line indicates their offspring. The genotype of individuals can be worked out by using a family tree.

Sex inheritance In humans, sex is determined by one of 23 pairs of chromosomes. These chromosomes are called

the sex chromosomes. The other 22 pairs are called autosomes. The sex chromosomes differ from the autosomes in that two sex chromosomes in a cell are not

always alike. They do not always have the same genes in the same position, and so they are not homologous.

There are two types of sex chromosomes, known as X and Y chromosomes because of their shapes. Y chromosome is much shorter than X, and carries few genes. A person with two genes is called a female and a person with one X and one Y chromosome is male.

Sex linkage-Hemophilia: The X chromosome contains many different genes. One of them is a gene that codes for the

production of a protein needed for blood clotting, called factor VIII. There are two alleles of this gene, the dominant one, H, producing normal factor VIII, and the recessive one, h, resulting in lack of factor VIII.

People who are homozygous for the recessive allele suffer from the disease hemophilia, in which the blood fails to clot properly.

The fact that the gene for hemophilia is on the X chromosome, and not on an autosome, affects the way that it is inherited. Females who have two X chromosomes have two copies of the gene. Males, however, who have only one X chromosome, have only one copy of the gene. Therefore, the possible genotypes for men and women are different.

The factor VIII gene is said to be sex linked. A sex linked gene is one that is found on a part of the X chromosome not matched by the Y, and therefore not found on the Y chromosome.


Genotypes including sex-linked genes are always represented by symbols that show that they are on an X chromosome. Thus the genotype of a woman who has the allele H on one of her X chromosome , and allele h on the other, is written as XHXh.

Red-green color blindness: A study of the crosses reveals that the recessive gene causing color

blindness is exchanged from one sex to the other at each generation. The father passes it to his daughters, who thus become carriers. The daughters in turn may pass it to their sons, who are thus color blind. This type of pattern is more obvious when viewed another way.

As the male is XY, his Y chromosome must have inherited from his father as the mother does not possess Y chromosome. The X chromosome and hence color blindness must therefore have been inherited from the mother.

The color blind male can only donate his X chromosome to his daughters as it is bound to fuse with another X chromosome- the only type the mother produces.

Color blind females can only arise from a cross between a carrier female and a color blind male.

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