More of MendelMore of MendelMore of MendelMore of Mendel

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What we know…• Inheritance of biological characteristics

is determined by genes• Principle of Dominance

– When there are two or more alleles for a gene, some are dominant while others are recessive

• Law of Segregation– In sexually reproducing organisms, adult

cells have two copies of each gene—one from each parent; these genes segregate when GAMETES are formed

Does the segregation of 1 pair of alleles affect the segregation of another

pair of alleles?

Make a test cross of:•True breeding Round Yellow Peas

•Genotype (RRYY)•True breeding Wrinkled Green peas

•Genotype (rryy)

RY

RY

RY

RY

ry ry ry ry

• What is the phenotype of the F1 offspring?

• What is the genotype of the F1 offspring?

What this first cross told us…

• All F1 offspring were heterozygous for seed shape (round) and seed color (yellow) RrYy

• The F1 plant was made from fusing a gamete carrying RY and a gamete carrying ry

• Will the dominant alleles stay together or separate when making the F2 offspring?

Now cross these hybrid (RrYy) plants on a new

4X4 Punnett Square

What does the F2 cross tell us?

• Are there combinations of alleles that we did not see in either of the parents?

• This means that the alleles for seed color separated separately than the alleles for seed shape

• Genes that segregate separately do NOT influence each other’s inheritance

Principle of Independent Assortment

• Genes for different traits can segregate independently during the formation of gametes. This principle helps account for many genetic variations in plants, animals and other organisms.

Mendel’s 4 Principle’s• Inheritance of biological

characteristics is determined by genes

• Principle of Dominance• Law of Segregation• Principle of Independent

Assortment

Exceptions to Mendel• Genetics more complicated• Some alleles are neither dominant

nor recessive• Many traits are controlled by

multiple alleles or multiple genes

Incomplete Dominance• When red flowered (RR) plants were

crossed with white flowered (WW) plants they made…pink flowers (RW)

• Which allele is dominant?– neither

• Incomplete dominance:– Case in which one allele is not dominant

over another– The heterozygous phenotype is somewhere

between the two homozygous phenotypes

Codominance• Both alleles contribute to the phenotype• Chickens

– Allele for black feathers is codominant with allele for white feathers

– Chicken looks speckled with black and white feathers– Not like the blending of dominant phenotypes…– BOTH dominant phenotypes show up

• In humans– Gene for protein that controls cholesterol levels in the

blood– People with heterozygous form make both types of

protien

Multiple Alleles• When a gene has more than two alleles• NOT more than 2 alleles for a person

but MORE than 2 alleles for the trait exist

• Coat color in rabbits– A single gene for coat color– At least 4 different alleles– Simple dominance and make 4 possible

coat colors• Genes for human blood type

Polygenic Traits• “Poly” many• “-genic” genes• Traits controlled by two or more genes• Several genes interact to produce a trait• Wide range of phenotypes• Skin color

– Four different genes

• Fruit Fly eye color– Three genes make the reddish brown pigment

Applying Mendel’s Principles…

• Apply Mendel’s Principles to many organisms, including humans

• Thomas Hunt Morgan (1900’s)– American geneticist– Common fruit fly

• Drosophilia melanogaster• Produced offspring very quickly• Single pair of flies=100 offspring

• Mendel’s principle’s were tested with Drosophilia and many other organisms and they applied to all of them as well

Genes and the Environment

• Genes provide the plan for development• How the plan unfolds depends on the

environment• Example:

– Sunflower has genes for height and color of flowers

– But these traits are also influenced by climate, soil conditions and water availability