Chapter 7: Mendelian Inheritance

Family resemblance: how traits are inherited

Lectures by Mark Manteuffel, St. Louis Community College

7.1 Family resemblance: your mother and father contribute equally to your genetic makeup.

Selective Breeding: Observing Heredity

Traits that are determined by the instructions a person carries at one gene are called single-gene traits.

More than 9,000 single-gene traits have been identified in humans.

7.3 Mendel learned about heredity by conducting experiments.

It’s easy to control fertilization in plants.

True-Breeding

True-breeding = offspring always have the same trait as their parents. Traits were easily observed.

Homologous chromosomes have alternative alleles.

• Homozygous for that gene…– When an organism has identical alleles for

a gene. (Ex. PP or pp)

• Heterozygous for that gene…– When an organism has different alleles for

a gene. (Ex. Pp)

Homologous chromosomes may bear either the same alleles or different ones at a given locus.

A dominant trait masks the effect of a recessive trait.

Dominant traits determine the phenotype.

Three Ideas Mendel Used for Explaining This Pattern of Inheritance1) Each parent puts into every sperm or egg it

makes a single set of instructions for building the trait.

2) Offspring thus find themselves with two copies of the instructions for any trait (called alleles).

3) The actual trait produced by an individual depends on the two copies of the gene that they inherit from their parents.

• homozygous and heterozygous

7.4 The Principle of Segregation: you’ve got two copies of each gene but put only one copy in each sperm or egg.Mendel’s Test: a monohybrid cross…

7.5 Observing an individual’s phenotype is not sufficient for determining its genotype. • Genotype

– An organism’s genetic makeup – designated by alleles (alternative forms of genes on homologous chromosomes)

• Phenotype– An organism’s physical traits – appearances,

may also manifest in behaviors.

• Assign letters to represent alleles… – Capital letters = dominant traits – determine

phenotype– Lower-case letters = recessive traits – may be

hidden

It is not always possible to determine an individual’s genotype from its phenotype.

You would like to produce white alligators via a mating program.

The problem is that you cannot be certain of the genotype of your alligators.

7.7 A test-cross enables us to figure out which alleles an individual carries.

They might be homozygous dominant, MM, or they might be heterozygous, Mm.

In either case their phenotype is normal coloration.

How can you figure out which of these two possibilities is the actual genotype?

In a test-cross, an individual with a dominant phenotype and an unknown genotype is mated with a homozygous recessive individual.

The phenotypes of the offspring reveal the unknown genotype.

7.8 Using pedigrees to decipher and predict the inheritance patterns of genes.

Pedigree: a type of family tree

Analyzing Which Individuals Manifest the Trait and Which Do Not

Probability has a central role in genetics for two reasons:1. The first is a consequence of segregation.

In segregation, each gamete receives only one of the two copies of each gene.

It is impossible to know which allele goes into which gamete.

2. The second reason is that fertilization, too, is a chance event.

All of an individual’s sperm or eggs are different. Any of these gametes may be the gamete involved in

fertilization.

7.6 Chance is important in genetics.

Punnett Squares are very useful in predicting probability.

7.9 Incomplete dominance and codominance: the effects of both alleles in a genotype can show up in the phenotype.

Incomplete dominance — a heterozygote displays a characteristic somewhere between the characteristics (a blended version of the trait) of the two homozygotes.

Codominance — a heterozygote displays characteristics of both homozygotes equally.

Incomplete dominance, in which the heterozygote appears to be intermediate between the two homozygotes.

• Hypercholesterolemia– Is a human trait that is incompletely

dominant

Figure 9.19

LDL (carriescholesterol)

LDL receptor(mops up LDL)

Cell

NormalHH

Mild diseaseHh

Heterozygous

Severe diseasehh

Codominance, in which the heterozygote displays characteristics of both

homozygotes.

Both phenotypes are expressed equally in the heterozygote.

7.10 What’s your blood type?

Some genes may have more than two alleles.

Multiple allelism, in which a single gene has more than two alleles. Each individual still carries only two alleles.

Inheritance of the ABO Blood Groups

A, B, and O alleles

The A and B alleles are both completely dominant to O.

The A and B alleles are codominant to each other.

Individuals can be one of four different blood types: A, B, AB, and O.

Why are people with type O blood considered “universal donors”? Why are those with type AB considered “universal acceptors”?

7.11 Multi-gene TraitsHow are continuously varying

traits such as height influenced by genes?

• Polygenic inheritance is the additive effect of two or more genes on a single phenotype.

• This hypothetical example shows a range of possible skin colors in the F2 Generation based on just 3 alleles.

Figure 9.22

aabbcc(very light)

AABBCC(very dark)

AaBbCc

Eggs Sperm

7.12 Pleiotropy: How can one gene influence multiple traits?

What is the benefit of “almost” having sickle cell disease?

7.13 Why are more men than women color-blind?

Sex-linked traits differ in their patterns of expression in males

and females.

If a man is color-blind, did he inherit this condition from his

mother, his father, or both parents?

Hemophilia is another sex-linked (X-linked) trait. Women are often carriers.

7.14 Environmental effects: identical twins are not identical.

Genotypes are not like blueprints that specify phenotypes.

Many phenotypes are a product of the genotype in combination with the environment.

Could you create a

temporarily spotted Siamese cat with an ice pack?

• Many human characteristics result from a combination of heredity and environment.– Eye color appears to be entirely genetic.– The height of an individual is partially

genetic, but can also be influenced by health and diet during childhood and adolescence.

– Often characteristics such as susceptibility to heart disease, cancer, alcoholism, and schizophrenia are influenced by both genes and environment.

The Role of Environment

The inheritance pattern of one trait doesn’t usually influence the inheritance of any other trait.

7.15 Most traits are passed on as independent features: Mendel’s law of independent assortment.

Mendel tested this using a dihybrid cross.

7.16 Red Hair and Freckles

Genes on the same

chromosome are sometimes inherited together.

The alleles for two genes may be inherited and expressed together when they are close together on the same chromosome.