modern genetics

Modern GeneticsPart 5

Gregor Mendel: “Father of Genetics” Austrian Monk (1822-1884) Published the results of scientific research on

Garden Peas (Pisium sativum) in 1865.

Why study peas? Great natural variation- stem length, seed

color, pod shape ,pod color, small, edible, easy to grow, many offspring, easy to cross fertilize

History

Trait: characteristic of an organism. Gene: piece of DNA that codes for a protein. Allele: different forms of a gene.

Genotype: The allele combination of an individual.

Phenotype: The visible characteristics that result from a genotype.

Useful Terms

I. Human ChromosomesA. Types

1. Sex Chromosomes – 1 pair – carry the genes that determine male and female features (also some non-sex traits)

1. X and Y do not look alike but behave as a homologous pair at meiosis

2. XX = female, XY = male2. Autosomes – non-sex chromosomes (22

pairs) – genes are unrelated to sex determination

Chromosomes and Human Genetics

PUNNETT SQUAREMethod for determining possible allele combination for the offspring

Gametes outside

Offspring Inside

Determining Sex…X X

X

Y

MOM

DAD

X X X X

X Y X Y

TallP Dwarfx

F1 All Tall

Phenotype

One Example of Mendel’s Work

Clearly Tall is Inherited…What happened to Dwarf?

F1 x F1 = F2

F23/4 Tall1/4 DwarfDwarf is not missing…just masked as “recessive” in a diploid state… there IS a female contribution.

1. Tall is dominant to Dwarf

2. Use D/d rather than T/t for symbolic logic

DD dd

Dd

GenotypeHomozygous

DominantHomozygous

Recessive

Heterozygous

Dwarfdd

TallDdd

TallDd

TallDDD

dDPunnett Square:

possible gametes

possible gametes

In hamsters, white fur color (W) is dominant to brown fur color (w). If you cross a heterozygous female with white fur color (Ww) with a male that has brown fur (ww), what genotypes and phenotypes would you see and in what ratios?

Sample Problem

Genetics After MendelRed

xYellow

All OrangeWhen these alleles go walking, they both do some talking (codominance)!

OK, so we cannot use R/r nor Y/y so we pick a third letter…P for the petal color gene.

Notice: we do NOT mix R/Y or r/y!

PRPR PYPY

PRPY

F1 x F1 = F2

F2YellowPYPY

Orange

PRPY

PY

Orange

PRPY

RedPRPRPR

PYPRPunnett Square:

possible gametes

possible gametes

P

F1

This F2 will NOT have a 3:1 ratio of phenotypes.

Instead it shows a 1:2:1 ratio!

The exception here proves the rule.

After 1900 several scientists tried to replicate Mendel’s crosses using other species including snapdragon.

In horses, black and white coat colors are codominant. Heterozygous horses have gray coats.

Black horse genotype: HBHB

White horse genotype: HWHW

Gray horse genotype: HBHW

What would be the possible genotypes and phenotypes of the following crosses?

Black x WhiteGray x Gray

Sample Problem

Blood Types

Blood Type:ABABO

Genotype IAIA , IAi IBIB , IBi IAIB ii

1. If a male is homozygous for blood type B and a female is heterozygous for blood type A, what are the possible blood types in the offspring?

Try These

2) Is it possible for a child with Type O blood to be born to a mother who is type AB? Why or why not?

Chromosomes and Human Genetics

I. Human ChromosomesA. Types

1. Sex Chromosomes – 1 pair – carry the genes that determine male and female features (also some non-sex traits)

1. X and Y do not look alike but behave as a homologous pair at meiosis

2. XX = female, XY = male2. Autosomes – non-sex chromosomes (22

pairs) – genes are unrelated to sex determination

B. Determining Sex…X X

X

Y

MOM

DAD

X X X X

X Y X Y

II. Gene LocationA. Linked – Linkage Groups – genes located so close

together on a chromosome that the traits always seem to appear together

Ex. Red hair and frecklesEx. Colorblindness and Hemophilia

X X

B. Sex-linked Traits – genes on the sex chromosomes

- Expression of certain genes often appears more in one sex than the other

- Males require only one copy of a gene since they only have one X chromosome

- See Royal Families of Europe Pedigree- Ex. Eye color in fruit flies, hemophilia, color-blindness

Colorblindness Tests

X-Linked/Sex Linkage – do not write

Genes present on the X chromosome exhibit unique patterns of inheritance due to the presence of only one X chromosome in males.

X-linked disorders show up rarely in females X linked disorders show up in males whose

mothers were carriers (heterozygotes)

Practice Sex-linked Problems…. What will the result of mating between a normal

(non-carrier) female and a hemophiliac male?

A female carrier who is heterozygous for the recessive, sex-linked trait causing red-green colorblindness, marries a normal male. What proportions of their MALE progeny will have red-green colorblindness?

Hemophilia is inherited as an X-linked recessive. A woman has a brother with this defect and a mother and father who are phenotypically normal. What is the probability that this woman will be a carrier if she herself is phenotypically normal?

C. Gene Interactions

1. Polygenetic trait – many genes influence a single trait (ex. Height, intelligence)

2. Pleiotropic effect – one gene having many effects (ex. Gene to make testosterone)

Pleiotropy Expression of a single gene has

multiple phenotypic effects Marfan Syndrome – abnormal

gene that makes fibrillin (important in connective tissues)

?

III. Genetic Analysis

A. Karyotype – visualized chromosomes stained, squashed, and photographed at metaphase

- They are characteristic of the species or individual

B. Pedigree – chart showing family relationships (see

worksheet)

Pedigree Analysis Method of tracking a

trait through generations within a family.

Good method of tracking sex-linked traits as well as autosomal traits.

Sex-Linked Pedigree Shows gender bias

with males exhibiting the trait more often than females

Autosomal Dominant Pedigree

Autosomal dominant traits do not skip a generation

Autosomal dominant traits do not show gender bias

Autosomal Recessive Pedigree

Autosomal recessive traits skip a generation

Autosomal recessive traits do not show gender bias

IV. Non-Mendelian Genetics

1. Incomplete Dominance – blended inheritance

- Neither form of the gene is able to mask the other

- Ex. Snap dragon petal color- R1R1 – RED- R1R2 – PINK- R2R2 - WHITE

Incomplete Dominance

Neither allele is dominant Heterozygotes are a blend of

homozygous phenotypes = no distinct expression of either allele

Try these In a plant species, if the B1 allele (blue flowers)

and the B2 allele (white flowers) are incompletely dominant (B1 B2 is light blue), what offspring ratio is expected in a cross between a blue-flowered plant and a white-flowered plant?

•What would be the phenotypic ratio of the flowers produced by a cross between two light blue flowers?

2. Codominance

No dominance and both alleles are completely expressed

Ex. Cat color C1C1 – Tan C1C2 – Tabby (black and tan spotted) C2C2 - Black

Try These1. Cattle can be red (RR = all red hairs), white

(WW = all white hairs), or roan (RW = red & white hairs together.

a. Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull.

b. What should the genotypes & phenotypes for parent cattle be if a farmer wanted only cattle with red fur?

1. A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together).

a. What pattern of inheritance does this illustrate?

b. What percent of kittens would have tan fur if a tabby cat is crossed with a black cat?

3. Multiple Alleles More than 2 alleles for one trait Ex. Eye color, hair color, blood type, guinea pig fur

color ABO blood groups

Each individual is A, B, AB, or O phenotype Phenotype controlled by marker on RBC IA and IB alleles are dominant to the i allele IA and IB alleles are codominant to each other

Blood TypesBlood Type:ABABO

Genotype IAIA , IAi IBIB , IBi IAIB ii

Try These1. If a male is homozygous for blood type B and a female

is heterozygous for blood type A, what are the possible blood types in the offspring?

2) Is it possible for a child with Type O blood to be born to a mother who is type AB? Why or why not?

3. A child is type AB. His biological mother is also type AB. What are the possible phenotypes of his biological father?

Human hair color follows a similar pattern:Alleles: HBn = brown HBd = blonde hR = red hbk = black

HBnHBn = dark brownHBnHBd = sandy brownHBnhR = auburnHBnhbk = dark brown

HBdHBd = blondeHBdhR = strawberry blondeHBdhbk = blonde

hRhR = redhRhbk = red

hbkhbk = blackDominant does NOT mean frequent! Recessive can

be common!