genetics part 2

More of MendelMore of Mendel

What we know…1. Inheritance of biological characteristics

is determined by genes2. Principle of Dominance

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

3. 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?

• Let’s look at Pea shape and pea color– Round (R) vs. wrinkled (r)– Yellow (Y) vs. green (y)

• Remember, there are 4 possible gametes• If a parent is homozygous dominant (true-

breeding dominant) for both shape and color, what will all possible gametes be?

• If a parent is homozygous recessive (true-breeding recessive) for both shape and color, what will all possible gametes be?

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 independently 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’s1. Inheritance of biological

characteristics is determined by genes

2. Principle of Dominance3. Law of Segregation4. 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 (CRCR) plants were

crossed with white flowered (CWCW) plants they made…pink flowers (CRCW)

• 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– 3 different alleles: IA, IB, I

• You can get different genotypes:– IAIA– IAi– IAIB– IBIB– Ibi– ii

• You can get different Phenotypes:– Type A (dom)– Type B (dom)– Type AB (dom)– Type O (recessive)

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

Epistasis• When the expression of one gene effects the expression of

another gene• Ex. Fur color in mice…controlled by 2 separate genes

– Gene 1• Brown fur pigment (BB or Bb) is dominant over gray

fur pigment (bb)– Gene 2

• Coat Pigment depositing gene • Dominant gene (CC or Cc) means fur will get pigment

and this is determined by gene one• Recessive gene (cc) means that no pigment will be

deposited on fur…whether the they have the gene for black or brown fur

• The gene for Pigment Deposition is the EPISTATIC gene because it alters the Phenotypic ration

• Sd

Homework• Complete a Dihybrid cross for 2

mice that are heterozygous for both Brown fur (Bb) and Pigment deposition (Pp)

• Predict your phenotypic ratio…• What are you final phenotypic

ratios?• Write a paragraph explaining your

results.

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

Genes

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

• Would genes on the same chromosome be inherited together?

• Thomas noticed that almost every single time he crossed two flies that each had red eyes and mini wings, the offspring almost always inherited BOTH red eyes and mini wings– This went against Mendel’s Principle

of Independent Assortment…(review!)

Gene Linkage• Thomas Hunt Morgan gave us the

answer– 1910 PRINCIPLE of LINKAGE– 50 Drosophilia genes– Seemed to contradict Principle of

Independent Assortment b/c certain genes were always inherited together

– He grouped the fly’s genes into linkage groups

• Linkage groups are made up of genes that seem to be inherited together

• Linkage groups assort independently but all genes in one group are inherited together

• Drosophilia– 4 linkage groups– 4 chromosomes

• What can be concluded?

Conclusions1. Each chromosome is actually a group of

linked genes2. Mendel’s Principle of Independent

Assortment holds true but an adjustment needs to be made…

IT IS THE CHROMOSOMES THAT ASSORT INDEPENDENTLY, NOT THE

INDIVIDUAL GENES

How Mendel missed it…

• What 7 genes did he study?• 6 of the 7 genes were on different

chromosomes• Two genes were on the same

chromosome but they were so far apart on the same chromosome that they assorted independently

• If two genes are on the same chromosome, does that mean they are linked for ever?– No, chromosomes cross

over during meiosis so they can separate

– Crossing over produces new combinations of alleles

– Important for genetic diversity

Lucky Student• Alfred Sturtevant 1911• Columbia University• Worked in Morgan’s Lab• Hypothesis

– If two genes are farther apart on a chromosome, the they are more likely to be separated during meiosis

• Experiment– Measured the rate at which linked genes were

separated and recombined to make a “map” of distances between genes

• Conclusion– Recombination rates could be used to make gene

maps– Gene maps showed the location of a gene on a

chromosome

Gene Linkage and Crossing Over

• The farther apart 2 genes are on a chromosome….– The more likely they are to “cross-over”

• The closer two genes are on the same chromosome…– The less likely they are to be separated

Steps for Dihybrid cross1. Make a Key

1. Trait 1: height1. Dominant phenotype: Tall TT or Tt (ways to get it) 2. Recessive phenotype: Short tt

2. Trait2: color1. Dominant phenotype: Purple PP or Pp2. Recessive phenotype: White pp 3. Write out genotypes for each parent

3. _ _ _ _ x _ _ _ _

2. Write Out Gametes for each Parent (use arrows)

1. 4 gametes for each parent (_ _)

3. Make Punnett Square (16) boxes1. Label parent one and write gametes along top2. Label Parent 2 and write their gametes on side3. Fill in each box (should have 4 letters)

4. Tally genotypes5. Write out 4 possible phenotypic combinations

1. Dominant trait 1 and dominant for trait 2:______2. Dominant trait 1 and recessive trait 2:_______3. Recessive trait 1 and Dominant trait 2:_______4. Recessive trait 1 and recessive trait 2:________

6. Tally Phenotypes (should =16)7. Write phenotypic ratio

1. __dd__:__dr__:__rd__:__rr__

• Dominant- capital letter D• Recessive- lower case d• Homozygous-

– 2 of the same size letter– If its two little letters

recessive dd– If its 2 big letters it is

dominant DD• Heterozygous-

– 2 different size letters (capital and lowercase) Dd

– ALWAYS dominant