Mendelian Patterns of Inheritance

Chapter 9

Introduction

• Gazelle always produce baby gazelles, not bluebirds

• Poppy seeds always produce poppies, not dandelions

Introduction

• Everyone who observes this phenomenon reasons that the parents must pass this hereditary information to their offspring

Introduction

• It also occurs that offspring can appear markedly different than either parent, however

• The laws of heredity must be able to explain not only the stability, but the variation that is observed between generations of offspring

Gregor Mendel: “Mendelian Genetics”

• Gregor Mendel, an Austrian monk in the 1860s, formulated two fundamental laws of heredity

• He is known as the Father of Genetics

Gregor Mendel

• Mendel experimented with the garden pea, Pisum sativum, in the gardens of the monestary, to test and formulate his hypotheses about inheritance

Mendel and Inheritance: “blending concept”

• Before Mendel, it was thought that both sexes contribute equally to an individual, and that parents of contrasting appearance should always produce offspring of intermediate appearance--the “blending concept” of inheritance

Blending concept: not always true!

• If this were the case, then crossing red and white flowers should always produce pink flowers

• We know that this is not always the case

• This discrepancy—when white and red flowers would show up in further generations—was explained by some instability in the breeding system

Mendel’s Experimental Procedure

• Mendel chose to work with the garden pea• They were easy to cultivate and had a short

generation time• They could easily be pollinated by hand• Many varieties were available

Mendel’s experimental procedure

• Mendel chose 22 varieties for his experiments

• When these varieties self-pollinated, they were referred to as “true breeding”—meaning that no offspring were like the parents and like each other

Some characteristics of the pea plants

Mendel’s experimental procedure

• Mendel studied simple and discrete traits of the peas—seed shape, seed color, and flower color

• He observed no intermediate characteristics among the offspring

One trait inheritance

• For his first experiment, Mendel crossed the tall and the short plant through cross pollination

One trait inheritance

• Mendel called the original parents the P generation

• He called the first-generation offspring the F1 generation

One trait inheritance: the first test cross

• If the blending theory were correct, the offspring should have the intermediate trait: all medium height plants

One trait inheritance

• His result of crossing the Tall and the Short plants: ALL TALL PLANTS! Not medium sized plants

• So, therefore, the F1 generation were all tall plants—resembling only one parent

• Did the characteristic for shortness disappear?

One trait inheritance

• Mendel then allowed these F1generation plants to self-pollinate with each other

• This next generation is referred to as the F2 generation

• The result?

One trait inheritance

• In the F2 generation, ¾ of the plants were tall, while ¼ of them were short, a 3:1 ratio

• So, the F1 plants were able to pass on the factor for shortness and it just didn’t disappear

• Perhaps the F1 plants were tall because tallness was dominant to shortness?

One trait inheritance

• Mendel explained why the short plants showed up in a 3:1 ratio in the F2 generation and not the F1 generation

• The F1 parents contained 2 separate copies of each hereditary factor, one being dominant and the other recessive

One trait inheritance

• These factors separated when gametes were formed, and each gamete carried only one gamete of each factor

• And random fusion of all possible gametes occurred upon fertilization

The Law of Segregation

• Each individual has 2 factors for each trait

• The factors segregate (separate) during the formation of gametes

• Each gamete contains only one factor from each pair of factors

• Fertilization gives each individual two factors for each trait

As viewed by modern genetics

• Each trait in a pea plant is controlled by two alleles, alternate forms of the gene—in this case, that control the length of the stem for tallness and shortness

• The dominant allele is so named because of its ability to mask the expression of the other allele, called the recessive allele

Dominance and recessiveness

• The dominant allele is identified by an uppercase (capital) letter

• The recessive allele is identified by a lowercase (small) letter

• So, in this reference, the allele for tallness (the dominant allele) is “T”, and the allele for shortness (the recessive gene) is “t”

alleles

• These alleles occur on a homologous pair of chromosomes at a particular location that is called the gene locus

alleles

• One allele for each trait is located in each gamete because of the division of chromosomes during gamete formation in meiosis

Homozygous alleles

• When an organism has two identical alleles, we say it is homozygous

• For instance, in Mendel’s P generation of Tall plants, the parents were homozygous for tallness: TT. The short plants were homozygous for shortness: tt

Heterozygous alleles

• When an organism has two different alleles at the same gene locus, we say that it is heterozygous

• Therefore, these F1 plants all had the alleles “Tt”

Genotype vs. Phenotype

• Two organisms with different allelic combinations for a trait can give the same outward appearance: for example, TT and Tt plants are both tall

• We distinguish between the alleles present in an organisms and the appearance of that organism

Genotype vs. phenotype

• The word genotype refers to the alleles an individual receives at fertilization

• Genotype may be referred to by letters or by short descriptive phrases

• Genotype TT is called homozygous dominant, and genotype tt is called homozygous recessive

• Genotype Tt is called heterozygous

Genotype vs. phenotype

• Phenotype refers to the physical appearance of the individual

• The homozygous dominant (TT) individual and the heterozygous (Tt) individual both show the dominant phenotype of being tall

• The homozygous recessive phenotype is short

Genotype vs. phenotype

Exceptions to simple Mendelian Inheritance

• 1) incomplete dominance: the offspring have an intermediate phenotype compared to the parents with two different phenotypes

• 2) multiple alleles: the offspring inherits 2 of several possible alleles

• 3) codominance: two inherited alleles are expressed equally

• 4) polygenic inheritance: This occurs when a single physical trait is governed by two or more sets of alleles

1. Incomplete Dominance

• Incomplete dominance is exhibited when the heterozygote has an intermediate phenotype between that of either homozygote

• For example (next slide), a red and a white flower will produce a pink flower

2) Multiple Allelic Traits

• When a trait is controlled by multiple alleles, the gene exists in several alleleic forms

• But, each person can only have two of the possible alleles

• Blood types are an example of this

Multiple Allelic traits and co-dominance: ABO blood types

• Three alleles for the same gene control the inheritance of ABO blood types

• These alleles determine the presence or absence of antigens on red blood cells:– IA = A antigen on red blood cells– IB = B antigen on red blood cells– i = neither A nor B antigen on red blood cells

3) Co dominance

• An example of co-dominance is:

• If a white flower and a red flower were crossed, the resulting offspring would be flowers with red and white stripes (not pink)

4) Polygenic inheritance

• This occurs when a trait is governed by two or more sets of alleles

• Each dominant allele has a quantitative effect on the phenotype, and these effects are additive

• The result is a continuous variation of phenotypes, resulting in a distribution that resembles a bell-shaped curve

• Skin color and height are examples of polygenic inheritance

Polygenic inheritance and epistasis

• Epistasis: this occurs when a gene at one locus interferes with a gene at a different locus

• Albinism is an example of this: no matter what genes for skin color are inherited from the parents, the gene for albinism interferes with the expression of alleles for skin color

Environment and phenotype

• Nutrition plays a part in height determination• Temperature can effect the color of primroses

and Himalayan rabbits• Soil acidity effects the color of certain flowers

Ex. hydrangea

Sex-linked inheritance

• We have two types of chromosomes:

• 1. sex chromosomes (X and Y: XX for a female, XY for a male)

• 2. autosomal chromosomes: all of our other chromosomes not including sex chromosomes

• Males produce 2 types of gametes: those that have an X and those that have a Y

• Females produce only one type of gamete: those that have an X

• Therefore, sex of the offspring is determined by the FATHER

Sex chromosomes

• Not only are sex-specific traits carried on sex chromosomes, but genes that have nothing to do with sex of the individual are carried here as well

• These are termed SEX-LINKED TRAITS

• i.e., X-linked traits are carried on the X chromosome