Modern Genetics Part 5

History 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

Useful Terms 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.

Chromosomes and Human Genetics Human Chromosomes Types Sex Chromosomes – 1 pair – carry the genes that determine male and female features (also some non-sex traits) X and Y do not look alike but behave as a homologous pair at meiosis XX = female, XY = male Autosomes – non-sex chromosomes (22 pairs) – genes are unrelated to sex determination

PUNNETT SQUARE Method for determining possible allele combination for the offspring Gametes outside Offspring Inside

Determining Sex… MOM X X X X X X X DAD X Y X Y Y

One Example of Mendel’s Work Tall x Dwarf Phenotype P DD dd Genotype Homozygous Dominant Homozygous Recessive All Tall Clearly Tall is Inherited… What happened to Dwarf? F1 Dd Tall is dominant to Dwarf Use D/d rather than T/t for symbolic logic Heterozygous F1 x F1 = F2 possible gametes Punnett Square: D d 3/4 Tall 1/4 Dwarf F2 D Tall DD Tall Dd possible gametes d Tall Dd Dwarf dd Dwarf is not missing…just masked as “recessive” in a diploid state… there IS a female contribution.

Sample Problem 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?

Genetics After Mendel P F1 F2 Red Yellow x PRPR PYPY All Orange PRPY After 1900 several scientists tried to replicate Mendel’s crosses using other species including snapdragon. Genetics After Mendel Red Yellow P x PRPR PYPY When 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! All Orange F1 PRPY F1 x F1 = F2 possible gametes Punnett Square: PR PY F2 PR Red PRPR Orange PRPY 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. possible gametes PY Orange PRPY Yellow PYPY

Sample Problem 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 White Gray x Gray

Blood Types Blood Type: A B AB O Genotype IAIA , IAi IBIB , IBi IAIB

Try These 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?

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 Human Chromosomes Types Sex Chromosomes – 1 pair – carry the genes that determine male and female features (also some non-sex traits) X and Y do not look alike but behave as a homologous pair at meiosis XX = female, XY = male Autosomes – non-sex chromosomes (22 pairs) – genes are unrelated to sex determination

B. Determining Sex… MOM X X X X X X X DAD X Y X Y Y

II. Gene Location Linked – Linkage Groups – genes located so close together on a chromosome that the traits always seem to appear together Ex. Red hair and freckles Ex. Colorblindness and Hemophilia X X

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 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 Polygenetic trait – many genes influence a single trait (ex. Height, intelligence) 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 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 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 These Cattle can be red (RR = all red hairs), white (WW = all white hairs), or roan (RW = red & white hairs together. Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull.

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

A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together). What pattern of inheritance does this illustrate? 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 Types Blood Type: A B AB O Genotype IAIA , IAi IBIB , IBi IAIB

Try These 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?

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

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

Recessive can be common! Human hair color follows a similar pattern: Alleles: HBn = brown HBd = blonde hR = red hbk = black HBnHBn = dark brown HBnHBd = sandy brown HBnhR = auburn HBnhbk = dark brown HBdHBd = blonde HBdhR = strawberry blonde HBdhbk = blonde hRhR = red hRhbk = red hbkhbk = black Dominant does NOT mean frequent! Recessive can be common!