Patterns of Inheritance · Inheriting Single Traits: Punnett Square •How to draw a Punnett...

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


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


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


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



• 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



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



• 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


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



• 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



B (brown

hair)

b (blonde

hair)

B (brown

hair)BB Bb

b (blonde

hair)Bb bb


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



• 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



• 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


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



• 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



• 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


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


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


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


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



• 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



• 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


Human Genetic Disorders: Hemophilia



• 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



• 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



• 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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