Chapter 11-3: Exploring Mendelian Genetics

Independent Assortment

To determine if the segregation of one pair of alleles affects the segregation of another pair of alleles, Mendel performed a two factor cross (dihybrid cross).

11-3

For example. . .

• Does the inheritance of flower color affect the inheritance of seed shape?

The Two-Factor Cross: F 1 • • Mendel crossed true-breeding plants that produced round yellow peas (genotype RRYY) with true breeding plants that produced wrinkled green peas (genotype rryy).

All of the F 1 offspring produced round yellow peas (RrYy).

11-3

The alleles for round ( ( Y wrinkled ( r R ) and yellow ) are dominant over the alleles for ) and green ( y ).

11-3

The Two-Factor Cross: F 2 • Mendel crossed the heterozygous F 1 plants (RrYy) with each other to determine if the alleles would segregate (separate) from each other in the F 2 generation.

RrYy

×

RrYy

11-3

The Punnett square predicts a 9 : 3 : 3 :1 phenotype ratio in the F 2 generation.

11-3

dihybrid 11-3

dihybrid

Phenotype

ratio from dihybrid cross is 16 16 16 16 or 9:3:3:1

11-3

Dihybrid phenotype ratio:

-9 of 16 show both dominant traits -3 of 16 show one dom., one recessive -3 of 16 show one rec., one dom.

-1 of 16 show both recessive

11-3

Law of independent assortment

•

During meiosis, genes for different traits are separated and distributed to gametes independently of one another

(if the genes are on different chromosomes).

11-3

The alleles for seed shape segregated independently of those for seed color. This principle is known as

independent assortment

.

Genes that segregate independently do not influence each other's inheritance.

11-3

Trihybrid!

A Summary of Mendel's Principles • • Genes are passed from parents to their offspring. If two or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive .

11-3

• In most sexually reproducing organisms, each adult has two copies of each gene. These genes are segregated from each other when gametes are formed.

• The alleles for different genes usually segregate independently of one another.

11-3

Incomplete Dominance

•

When both alleles contribute to the phenotype of a heterozygous individual to produce a trait not exactly like either parent.

11-3

A cross between red (RR) and white (WW) four o’clock plants produces pink-colored flowers (RW).

WW RR

1: 2: 1

Think of pink as an “incomplete” red color

Codominance

•

2 dominant alleles are expressed at the same time. They both contribute to the phenotype.

11-3

Codominance: roan fur coat 11-3

Codominance: Human blood types

3 Alleles: I

A

, I

B

, i

6 genotypes: I

A

I

A

, I

A

I

B

, I

A

i, I

B

I

B

, I

B

i, and ii

11-3

Having more than 2 alleles is called Multiple alleles

Multiple alleles:

ABO Blood Group System

Genotype I

A

I

A

or I

A

i I

B

I

B

or I

B

i I

A

I

B

ii Blood Type A B AB O

11-3

Polygenic Traits

• • Traits controlled by two or more genes are said to be polygenic

traits.

Skin color in humans is a polygenic trait controlled by more than four different genes.

11-3

Polygenic Traits

Human Skin Color

One more thing:

X Inactivation in female mammals

http://arbl.cvmbs.colostate.edu/hbooks/gene tics/medgen/chromo/mosaics.html

In a cross involving two pea plant traits, observation of a 9 : 3 : 3 : 1 ratio in the F 2 generation is evidence for • • the two traits being inherited together.

an outcome that depends on the sex of the parent plants.

• the two traits being inherited independently of each other.

• multiple genes being responsible for each trait.

• • • Traits controlled by two or more genes are called • multiple-allele traits.

polygenic traits.

codominant traits.

hybrid traits.

In four o'clock flowers, the alleles for red flowers and white flowers show incomplete dominance. Heterozygous four o'clock plants have • pink flowers.

• • white flowers.

half white flowers and half red flowers.

• red flowers.

• • • A white male horse and a tan female horse produce an offspring that has large areas of white coat and large areas of tan coat. This is an example of • incomplete dominance.

multiple alleles.

codominance.

a polygenic trait.

Mendel's principles apply to • pea plants only.

• • • fruit flies only.

all organisms.

only plants and animals.

Chimeras: combination of two very early embryos such that their cells intermix and the resulting “conceptus” has cells from both original embryos http://arbl.cvmbs.colostate.edu/hbooks/genetics/medgen/chromo/mosaics.html

Chimeric mice and sheep-goat chimeras have been most useful in answering fundamental questions about developmental biology and pathology. There is also some potential that this technique can be applied to problems such as rescue of endangered species. It is possible, for example to construct a goat-sheep chimera such that a goat fetus is "encased" in a sheep placenta. This enables a sheep to carry a goat to term, which will not occur if you simply transfer goat embryos into sheep (the sheep will immunologically reject the goat placenta and fetus). It may be possible to extend this procedure to allow embryos from severely endangered species to be carried by recipient mothers from another species.