patterns of inheritance · inheriting single traits: punnett square •how to draw a punnett...
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Patterns of Inheritance
Introduction to Life Processes - SCI 102 1
Lesson 7
Physical Basis of Inheritance
• Inheritance: the process by which characteristics of
individuals are passed to their offspring
• Genes are sequences of nucleotides at specific
locations on chromosomes
Genes are the units of inheritance
A gene’s location on a chromosome is called its locus
Genes occur as pairs of alleles
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Physical Basis of Inheritance
• Mutations: the source of alleles
Mutations are changes in the DNA sequence of a gene
• An organism’s two alleles may be the same or
different
If both homologous chromosomes have the same allele,
the organism is homozygous at that locus
If both homologous chromosomes have different alleles,
the organism is heterozygous at that locus
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Discovery of Principles of Inheritance
• Gregor Mendel deduced the common patterns of inheritance
• Doing it right: the secrets of Mendel’s success
Mendel chose pea plants as the subjects for his experiments
• Reproductive structures are enclosed within petals
This normally prevents cross-pollination
Pea plants, however, often get self-fertilized, where a flower on one plant
pollinates itself
• Mendel artificially caused cross-pollination
Self-pollination is normal, simplifying self crosses
True-breeding varieties were already available
Mendel chose to examine single traits individually
Mendel followed traits through several generations
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Inheriting Single Traits
• Mendel’s early experiments involved flower color: purple and
white true-breeding plants
In the parental generation (P), one parent was true-breeding for purple
and the other true-breeding for white
• True-breeding parent plants always produce the same color flower each time
they reproduce
The first filial generation (F1) was the offspring
• All were purple
Mendel self-fertilized the F1 generation to produce the F2 generation
• White flowers reappeared among the F2
• About ¾ were purple and ¼ white
The white trait had “receded” into the background
The purple trait had “dominated” white
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Inheriting Single Traits
• The inheritance of dominant and recessive alleles on
homologous chromosomes can explain the results of Mendel’s
crosses
Each trait is determined by discrete pairs of physical units called
genes
When two different alleles are present in an organism, the dominant
allele may mask the expression of the recessive allele
Pairs of alleles on homologous chromosomes segregate from each
other during meiosis
• This is Mendel’s law of segregation
The distribution of alleles into gametes is random
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Inheriting Single Traits
True-breeding plants have two copies of the same allele for a
given gene and are homozygous
• Hybrid organisms have two different alleles for a given gene and
are heterozygous
Mendel’s experiments with flower color provided the evidence
for his conclusions
• Parents that were true-breeding and homozygous for each flower
color produced heterozygous offspring, the F1 generation
• Mating the F1 individuals together produced an F2 generation with
a 3:1 ratio of purple to white flowers
Genotype: the combination of alleles carried by an individual
Phenotype: the physical appearance of the individual
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Inheriting Single Traits
• Simple “genetic bookkeeping” can predict
genotypes and phenotypes of offspring
The Punnett square method is a convenient way for
following alleles during crosses
• Mendel’s hypothesis can be used to predict the
outcome of new types of single-trait crosses
A test cross is a cross between a dominant phenotype
and a recessive phenotype
• It can determine the genotype of the dominant individual
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Inheriting Single Traits: Punnett Square
• Punnett squares are used for:
Figuring out the likelihood of gene
expression
Figuring out what alleles an organism has
Figuring out which traits are dominant and
recessive
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Inheriting Single Traits: Punnett Square
• How to draw a Punnett square:
1) Draw a box with four squares
2) Outside the box, on the top, write the father’s genes
• Use an uppercase letter for a dominant gene and a lowercase
letter for recessive genes
3) Repeat step 2 for the mother, this time to the left of the
box
4) Write the corresponding letters inside the four squares
5) Conclusion will be based on the results in the boxes
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Inheriting Single Traits: Punnett Square
B (brown
hair)
b (blonde
hair)
B (brown
hair)BB Bb
b (blonde
hair)Bb bb
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Conclusion: a child that will be born from these parents will have a 75%
chance of having brown hair and a 25% chance of having blonde hair
Inheriting Multiple Traits
• After figuring out the inheritance of single traits,
Mendel turned his attention to the inheritance of
multiple traits in peas
Mendel crossed plants differing in two traits: seed color
and seed shape
F1 individuals all showed the dominant trait for each
gene
He saw a 9:3:3:1 phenotype ratio among F2 offspring
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Inheriting Multiple Traits
• Mendel hypothesized that traits are inherited
independently
Mendel realized that his results could be explained if the
traits for seed color and seed shape were inherited
independently from each other
• His results supported this idea
• This is the law of independent assortment
Multiple traits are inherited independently because the alleles of one
gene are distributed to gametes independently of the alleles for
other genes
The events of meiosis explain independent assortment
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Inheriting Multiple Traits
• In an unprepared world, genius may go
unrecognized
Mendel presented his findings in 1865, but they were
largely misunderstood
It was not until 1900 that Mendel’s work was
“rediscovered” and he got the credit he deserved
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Rules of Inheritance
• A review of Mendel’s assumptions indicates that:
Traits are controlled by a single gene
There are two alleles for each trait
One allele is completely dominant over the other
There are other possibilities, however
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Rules of Inheritance
• In incomplete dominance, the phenotype of heterozygotes is
intermediate between the phenotypes of the homozygotes
Curly hair in humans is an example of incomplete dominance
• A single gene may have multiple alleles
Examples in humans include Marfan syndrome, Duchenne muscular
dystrophy, and cystic fibrosis
Another example is human ABO blood groups
• There are six possible genotypes
• Both A and B are dominant to O
This is known as codominance
• The biochemical explanation involves antibodies to cell-surface antigens
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Rules of Inheritance
• Many traits are influenced by several genes
This is an example of polygenic inheritance
Human skin color is an example of this type of inheritance
• Genes typically have multiple effects on phenotype
This is called pleiotropy, where single genes affect more than one
phenotypic trait
An example is the nude mice mutation
• The environment influences the expression of genes
In Siamese cats, temperature affects the expression of fur color
Many traits are probably influenced by the environment
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Gene Location
• Genes on the same chromosome tend to be inherited together
Such genes are said to exhibit gene linkage
An example of flower color and pollen shape in sweet peas shows
linkage
• Crossing over creates new combinations of linked alleles
Crossing over during meiosis explains genetic recombination
• This is the appearance of new combinations of alleles that were previously
linked
• This process occurs during prophase I of meiosis
The distance between genes on a chromosome determines the
likelihood of crossing over
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Inheriting Sex and Sex-Linked Traits
• Sex chromosomes determine sex in many different types
of animals
Females have two X chromosomes
Males have one X and one Y chromosome
• The Y is much smaller than the X
• These pair up during prophase and act as homologues
• In mammals, the sex of an offspring is determined by
the sex chromosome in the sperm
The sex chromosome carried by sperm determines sex in
organisms in which males are XY
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Inheriting Sex and Sex-Linked Traits
• Sex-linked genes are found only on the X or only on the
Y Chromosome
Genes located only on sex chromosomes are referred to as
sex-linked genes
• There are few genes on Y, mostly for sex determination
• The X chromosome has many genes that determine traits in both
sexes
Males carry only one copy of genes on the X chromosome
• They will always express a recessive sex-linked trait
• Red-green color blindness is a common sex-linked trait
• Males inherit sex-linked traits from their mothers
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Human Genetic Disorders
• It is unethical to perform experimental crosses on humans
• Some human genetic disorders are controlled by single
genes
Some human genetic disorders are caused by recessive alleles• An individual usually must be homozygous to have the condition
• Heterozygotes are called carriers
• Albinism results from a defect in melanin production
• Sickle-cell anemia is caused by a defective allele for hemoglobin
synthesis
The red blood cells can become misshapen and do not function
properly
Sometimes heterozygotes have a mild form of disease
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Human Genetic Disorders
• Some human genetic disorders are controlled by
single genes Some human genetic disorders are caused by dominant
alleles • Huntington disease is caused by a defective protein that kills
cells in specific brain regions
Some human genetic disorders are sex-linked• These will be expressed in males more often than in females
• These traits frequently skip generations
• Examples include red-green color blindness, muscular dystrophy,
and hemophilia
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Human Genetic Disorders
• Based on pedigrees, scientists can figure out:
Who was a carrier for a disease
• They had the gene but it was not expressed
Who was not a carrier
• They can’t pass the disease on to their children
Who had the disease
• Example: Using the pedigree on the next slide:
Louis IV – Grand Duke of Hesse-Darmstadt: was not a carrier of the
disease
Alice – Princess of Hesse: was a carrier of the hemophiliac disease
Leopold – Duke of Albany: was a hemophiliac male, meaning he had
two recessive genes for the disease
Introduction to Life Processes - SCI 102 23
Human Genetic Disorders: Hemophilia
Introduction to Life Processes - SCI 102 24
Human Genetic Disorders
• Some human genetic disorders are caused by
abnormal numbers of chromosomes
Occasionally, errors in meiosis produce gametes with too
many or too few chromosomes
• These disorders are caused by nondisjunction of chromosomes
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Human Genetic Disorders
• Some genetic disorders are caused by abnormal numbers of sex chromosomes Nondisjunction of sex chromosomes may result in gametes with different
combinations of sex chromosomes
• Sperm: XX, XY, YY, or O
• Eggs: XX, O
Turner syndrome (XO)
• Individuals with this disorder are phenotypically female, but fail to go through puberty
• They are usually sterile and have other traits
Trisomy X (XXX)
• These individuals are female, and are usually fertile
Klinefelter syndrome (XXY)
• These individuals are phenotypically male and are usually sterile
Jacob syndrome (XYY)
• These individuals are male and have a variety of traits
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Human Genetic Disorders
• Some genetic disorders are caused by abnormal
numbers of autosomes
The loss of one autosome is usually fatal
The gain of one autosome often causes abortion, but
some survive
Trisomy 21 (Down syndrome)
• These individuals have three copies of chromosome 21
• The probability of this increases with the age of the parents
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