Variation: alleles & mutations.Genotype, phenotype & crosses.

CHAPTER 13 &14

A trait is a particular characteristic or feature of an organism. Eg. Blonde hair.

We inherit many physical features or traits and can also inherit diseases or defects.

These traits or characteristics are coded by particular genes located on particular and specifi c chromosomes.

TRAITS & INHERITANCE

• Chromosome images are organised by international convention, to assist in analysis.

• Homologous chromosomes are arranged in pairs according to their relative size, position of the centromere and banding patterns.

• Autosomes are identified by numbers in order of decreasing size.

• The pair of chromosomes that differ between the sexes is designated as the sex chromosomes.

THE KARYOTYPE

Human male karyotype

Human female karyotype

Autosomes

• Autosomes are the matched pairs of chromosomes that are present in males and females.•The number of genes carried on chromosomes vary.•Genes carried on the same chromosome are said to be linked.

• Genes locate don the X chromosome are said to be X-linked.

• Females have two alleles of a particular gene whereas males will only have one (as they only have one X chromosome).

• This accounts for why many X-linked diseases show up more often in males than females.

• Genes on the Y are Y-linked. • In mammals it is the male

that determines the sex of the off spring.

HUMAN SEX CHROMOSOMES

The Y chromosome is a degenerating X chromosome.

SEX DETERMINATION

AllelesGenes occupying the same position (locus) on homologous chromosomes are called alleles.

Alleles are versions of the same gene that code for a variant of the same polypeptide.Any one individual can only have a maximum of two alleles for a given gene.There may be more than two alleles in a population, e.g blood groups A, B, O.

Gene A

Genes that occupy the same locus code for the same trait.Gene

B

Gene C

Paternal chromosome

Maternal chromosome

Pod color in peasis a trait controlled by a single gene. The allele for green pods is dominant over the allele for yellow.

When both chromosomes have identical copies of the dominant allele for a gene, the organism is said to be homozygous dominant for that gene.

AllelesThese two different versions of gene A create a condition known as heterozygous. Only the dominant allele (A) will be expressed.When both chromosomes have identical copies of the recessive allele for a gene, the organism is said to be homozygous recessive for that gene.

Genes occupying the same locus or position on a chromosome code for the same trait and are said to be alleles.

Paternal chromosome that originated from the sperm of this person's father.

Maternal chromosome that originated from the egg of this person's mother.

A mutation is a heritable change in genetic material.

Mutations are sources of new alleles.

Mutations in genes can involve changes in small parts of the DNA sequence.

MUTATION

A change in a gene which is unpredictable is called a gene mutation.

Some mutations occur in normal body cells or somatic cells.

They often have no effect on the individual. But sometimes they produce a lump

of cells called a tumour. If a tumour spreads it is known as

malignant and the diseases caused by such tumours are cancer.

Mutations

Mutations in somatic cells cannot be passed on to offspring by sexual reproduction.

If the mutation is in the testes or ovaries, known as germ line cells then the mutated gene can be passed on.

There are two ways in which DNA can become mutated:Mutations can be inherited.

Parent to childMutations can be acquired.

Environmental damageMistakes when DNA is copied

Mutations can occur spontaneously

WHAT CAUSES MUTATIONS?

Point mutations occur due to the alteration of a single base in DNA. The following are point mutations.

Silent mutation results in a change in the DNA sequence which does not result in an amino acid change. This is due to the fact that some amino acids are coded by multiple triplet codes.

Missense mutations occur when a single substitution that results in the replacement of the amino acid. The consequences of this type of mutation depend on the protein involved.

Mutations that result in the generation of a stop codon are termed nonsense mutations. These mutation result in the stop codon (UAU or UAA). These can have severe consequences.

TYPES OF MUTATIONS

SUBSTITUTION MUTATION (MISSENSE)

Frameshift mutations: If a base pair is added to or deleted from the DNA, the wrong amino acids are incorporated for the rest of the sequence. This is called a frameshift mutation.

Genotype is the actual double set of genes (alleles) possessed by the individual. One from each of the homologous pairs in a diploid cell.

The number of possible genotypes depends on the number of allelic forms of the gene.

Think about:Hair colour in humansEye colour in humansFeather colour in budgiesAll are very varied so must have many alleles in the

population .

GENOTYPE

Phenotype is the physical expression of the inherited trait. The expression may be a physical, biochemical or physiological trait.

A dominant phenotype is a trait which requires only a single copy of the responsible allele for its phenotypic expression. It will expressed in heterozygous individuals. A dominant phenotype is represent by a capital lett er.

A recessive phenotype is a trait which expressed only in homozygous individuals. A lowercase lett er is used to represent recessive phenotypes.

Genotypes: BB = homozygous ( BB and Bb will have the same Bb = heterozygous phenotype. ) bb = homozygous

PHENOTYPE

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MENDEL’S LAWS

1. LAW OF DOMINANCE2. LAW OF SEGREGATION3. LAW OF INDEPENDENT

ASSORTMENT

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LAW OF DOMINANCE

In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation.

All the offspring will be heterozygous and express only the dominant trait.

RR x rr yields all Rr (round seeds)

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PUNNETT SQUARE

Used to help solve genetics problems

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LAW OF DOMINANCE

DOMINANCE AND RECESSIVE

A trait is dominant when the heterozygote and one homozygote have the same phenotype. A trait is recessive when the phenotype is observed only in one homozygote

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LAW OF SEGREGATION

During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other.

Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring .

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APPLYING THE LAW OF SEGREGATION

1. Every trait (like flower color, or seed shape, or seed color) is controlled by two "heritable factors". [We know now that these are genes - we each have two copies of every gene].

  2. If the two alleles differ, one is dominant (will be

observed in the organisms appearance or physiology) and one is recessive (cannot be observed unless the individual has two copies of the recessive allele). Dominant traits mask the appearance of recessive traits.

3. Alleles are randomly donated from parents to offspring - the factors (alleles) separate when the gametes are formed by meiosis, allowing all possible combinations of factors to occur in the gametes.

DOING A GENETIC CROSS (MONOHYBRID = 1 GENE):

Geneticists use letters be used to represent alleles.

A capital letter = Dominant trait, a lowercase letter = a recessive trait. The same letter is used to indicate both

alleles. Examples

= Flower color: P= purple, p= white = Seed color: Y= yellow, y = green = Seed shape: W = wrinkled, w = round

In humans... = Widow's peak: W = widow's peak, w = continuous

hairline = Freckles: F = Freckles, f = no freckles (which are

you?) = Earlobes: E = unattached, e = attached (which are

you?) = Cystic fibrosis C = no CF, c = cystic fibrosis

E-ZY STEPS FOR DOING GENETICS PROBLEMS:

1. Indicate the genotype of the parents using letters 2. Determine what the possible gametes are   3. Determine the genotype and phenotype of the

children after reproduction. To consider every type of offspring possible, use a Punnett Square in which all possible types of sperm are lined up vertically and all types of eggs are lined up horizontally:

4. Fill in the squares by "multiplying" the alleles from male & female.

GENETICS PROBLEM 1:

(a) A man with a widow's peak (WW) marries a woman with a continuous hairline (ww). A widow's peak is dominant over a continuous hairline. What kind of hairline will their children have?

GENETICS PROBLEM 1:

1. P1 Widow's peak (WW) x continuous hairline (ww)

2. Gametes: Male: W only, Female: w only

3. Children: (the F1 generation):

P1 WW x ww

Gametes W w

F1 Ww - Ww

Genotype – Ww (heterozygous)

Phenotype- widows peak all children

(B) SUPPOSE ONE OF THEIR CHILDREN (WW) MARRIES SOMEONE WHO IS ALSO HETEROZYGOUS

(WW). WHAT TYPE OF HAIRLINE WILL THEIR CHILDREN

HAVE?P1 Ww x Ww

Gametes W w male W w female

Genotype: Their children have a 25% (1/4) chance of being WW, a 50% (2/4) chance of being Ww, and a 25% (1/4) chance of being ww. (Note that this is a 1:2:1 genotypic ratio IF both parents were heterozyhous to begin with)

Phenotype: Their children will have a 3/4 chance of having a widow's peak and a 1/4 chance of having a continuous hairline (3:1 phenotypic ratio)

W w

W WW Ww

w Ww ww

MORE PROBLEMS

Genetics problem 2: A man and a woman are heterozygous for freckles. Freckles (F) are dominant over no freckles (f). What are the chances that their children will have freckles?

Genetics problem 3: A woman is homozygous dominant for short fingers (SS). She marries a man who is heterozygous for short fingers (Ss). Will any of their children have long fingers (ss)? yes / no

2 GENE INHERITANCE

So far we have looked at inheritance of a single gene. This is called monohybrid.

Inheritance can involve two genes: for example eye color and hair color. We call this dihybrid.

REVIEW

Gametes (sex cells) only receive one allele from the original gene.

As you know, 2 alleles control a gene. "Ee" is a gene in the critter opposite. When gametes are produced, the alleles of the gene separate and go into different sex cells; in other words, one letter is packaged in one sex cell and the other letter is packaged in another. Please note that each sex cell contains 50% of the original gene.

Ee = gene of the Hybrid parent.

PRINCIPLE OF INDEPENDENT ASSORTMENT

Independent assortment occurs because in meiosis the segregation of one pair of homologous chromosomes (and the alleles they carry) does not influence the segregation of other homologous pairs of chromosomes

A ratio of 9:3:3:1 will be observed if the following conditions apply:

Two genes control two traitsThere are two alleles for each geneFor each trait one phenotype is dominantBoth genes are on autosomesThe two genes assort independently

THE LAW OF INDEPENDENT ASSORTMENT

It appears that the inheritance of seed shape has no influence over the inheritance of seed colour

The two characters are inherited INDEPENDENTLYThe pairs of alleles that control these two characters assort

themselves independentlyThe pairs of chromosomes could orientate in different ways at

Anaphase 1

RULE OF INDEPENDENT ASSORTMENT

All this means is that the random selection of one trait will not determine the random selection of another. (This of course assumes that the genes are on separate chromosomes.) In other words, the genes for your eyes are transmitted independently of the genes for your height. Your kids could be tall with brown eyes. Or, your kids could be tall with blue eyes. The traits, or the alleles, assort independently of one another.

DIHYBRID CROSS

Mendel discovered independent assortment from following two traits simultaneously. If the traits are caused by genes on different chromosomes, they will be inherited independently of one another. If the first cross is between individuals homozygous for both traits the resulting offspring are heterozygous for both traits and are called dihybrids. A cross between these offspring is a dihybrid cross.

• So what is the chance that we can inherit two genes from a parent which are located on a different chromosomes together.

We work this out in the same way as monohybrid crosses by using a punnett square.

ALLELIC NOTATIONS

First we need to assign allelic notations to the genotypes. Or in plain terms ‘symbols’.

Gene 1 - B = Brown eyes b= blue eyesGene 2 – H = Black hair h = Blonde hair

To work out if the genes are on different chromosomes or if they are on the same chromosome you need to perform a cross.

THE CROSS

The parental generation will be homozygous x homozygous

dominant recessive.BBHH x bbhh

Brown eyes black hair x blue eyes blonde hair

This is the Parental generation.

The first step is to work out the gametes that the parents pass to F1 generation

GAMETES

BBHH The gametes for this parent are the different combinations of B and H you can have. No matter how you arrange these you will always get the same BH. Remember that Gametes have half the chromosome number (haploid) that the parents will have. So they will pass on half the letters.

What will the gametes be for bbhh?

CROSS ONE

To work out the F1 generation we cross the gametes from each parent.

BH x bhSo all the F1 generation will result in the same genotypeBbHhBrown eyes and Black Hair

BH BH

bh BbHh BbHh

bh BbHh BbHh

CROSS TWO

We then take F1 generation and self cross them. To do this we need to again work out the possible

gametes that they have. In this case it is every combination of both genes that includes at least one from each. So you must have either a B or b and a H or h. It also reduces the chromosome number by half.

The gametes will be the same for bothBH, bH, Bh, bh.You now put these into a punnett square to get the F2

generation.

BH bH Bh bh

BH

bH

Bh

bh

BH bH Bh bh

BH BBHH BbHH BBHh BbHh

bH BbHH bbHH BbHh bbHh

Bh BBHh BbHh BBhh Bbhh

bh BbHh bbHh Bbhh bbhh

RATIOS

So the F2 generation will produce a 9:3:3:1 ratio in the phenotype. This means that the inheritance is two gene on different chromosomes with a dominant gene for each one.

BH bH Bh bh

BH BBHH BbHH BBHh BbHh

bH BbHH bbHH BbHh bbHh

Bh BBHh BbHh BBhh Bbhh

bh BbHh bbHh Bbhh bbhh

P Phenotypes

Round Yellow seed

X Wrinkled Green seed

(Pure bred)

Genotypes

RRYY rryy

meiosis meiosis

Gametes RY ry

fertilisation

F

1

Phenotypes

RrYy (Selfed)

Genotypes

Round Yellow

Proportions

100%

DIHYBRID CROSS GENETIC DIAGRAM

© 2007 Paul Billiet ODWS

DIHYBRID CROSS

Dihybrid Cross. AA or Aa =

purple; aa = whiteBB or Bb = tall;

bb = short

Incomplete dominance

• With incomplete dominance, the phenotype is only partially expressed in the heterozygote.

• The phenotype of the heterozygote is different from either homozygote.

• A form of intermediate inheritance in which heterozygous alleles are both expressed, resulting in a combined phenotype.

For example, in cross-pollination experiments between red and white snapdragon plants the resulting offspring are pink.

Flower colour in snapdragons is an example of incomplete dominance. So what is wrong with how the genotypes have been expressed in this diagram?

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INCOMPLETE DOMINANCE

F1 hybrids have an appearance somewhat in between the phenotypes of the two parental varieties.

red (RR) x white (rr)

RR = red flowerrr = white flower

r

r

R R

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INCOMPLETE DOMINANCE

Rr

Rr

Rr

Rr

r

r

R R

All Rr = pink(heterozygous pink)

produces theF1 generation

CO-DOMINANCE

Both the alleles can be expressed

Eg. Red cows crossed with white will generate Roan cows.

Might seem to support blending theory.

Codominance

• Codominance occurs when the full phenotypic expression of both alleles is observed in the heterozygote

• Eg Blood types: where there are three

allelic forms at the same locus and individuals can have A, B, AB or O phenotypes.

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Codominance• Two alleles are expressed in heterozygous

individuals.• Example: blood type

• 1. type A = IAIA or IAi• 2. type B = IBIB or IBi• 3. type AB = IAIB

• 4. type O = ii

• In humans, there are four blood types: A,B,AB and O

• Blood type is controlled by three alleles: A,B,O• O is recessive, two O alleles must be present for the

person to have type O blood• A and B are Codominant. If a person receives an A

allele and B allele, their blood type is AB type• Crosses involving blood type often use an ‘I ‘ to

denote the alleles

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Summary of Dominance Relationships

X

Dominance

complete

X incomplete

X codominance

Environment affects somephenotypes

• While the phenotype is subject to inputs from genes (genotype), it may also be affected by the environment.

• Thus, if an individual with a given genotype develops in one environment, its phenotype may be different than if it had developed in some other environment.

•‘Identical’ but different twins:• For any given trait, differences in phenotype between

identical twins cannot be due to differences in genotype and must be attributed to differences in the environment,

• for example the effects of different diets, different exercise patterns and different climates.