Mendelian Genetics Review!

• Genetic Terms:

• P = parental generation of a cross

• F1 = (first filial) the first generation after the parental (the results of the first cross)

• F2 = (second filial) the offspring from crossing two F1’s

Mendel’s Law of Segregation• Alleles = alternative forms of the same gene

(P= purple, p= white)• Each individual has 2 alleles for every gene• Each parent passes one allele for each gene

to his/her offspring• What process does this law represent?• Meiosis• What stage of meiosis?• Anaphase I

Genetic terms

• Homozygous = 2 identical alleles for a gene

• Heterozygous = 2 different alleles for a gene

• Dominant = The allele that is expressed in the heterozygous individual

• Recessive = the allele that is masked in the heterozygous individual

Setting up the punnett square• The possible gametes of each individual

are determined.• Each gamete will have one of the two

alleles of the parent.• Generate a punnett square and write the

alleles for parent 1 on the left side of the Punnett square.

• Place the alleles for parent 2 above the square

• Fill the squares for each parent.

Interpreting the results of a Punnett square

• Genotypes that resulted from this monohybrid cross (Ss x Ss)

•

The test cross• How can you determine the genotype of a

dominant individual?

• In sheep, white is dominant to black

• It is customary to determine the letter for the alleles of a given gene by using the first letter of the dominant allele.

• Therefore W = white and w = black

• How can you tell if white sheep is homozygous or heterozygous?

• Conduct a test cross, where the unknown dominant individual is actually mated with the known recessive .

• W ___ x ww

• After mating, look at the offspring’s phenotype to determine the unknown parent’s genotype.

Dihybrid Cross

• R = red r = white

• T = tall t = short

• What is the F2 phenotypic ratio of offspring of a cross between pure red, tall plants and pure white, short plants?

• Punnett Square Approach

Law of Independent Assortment

• What a parent gives for flower color has nothing to do with what a parent gives for plant height. Genes are assorted independently of each other.

• What assumption did Mendel make for this law to work?

• The traits occur on different chromosomes!

Incomplete Dominance

• One allele is not completely dominant over the other, so the heterozygote has an intermediate phenotype between the 2 homozygotes

• In snapdragons, red snapdragons crossed with white will produce all pink flowers.

• Write your alleles like thisR’= redR= white

RR’= pink

Codominance

• Both alleles are dominant and are expressed equally

• A black chicken and a white chicken will produce a checkered chicken

• Write your alleles like this – B= black– W= white– BW= checkered

ProbabilityRule of multiplication

• The odds of 2 or more independent events occurring together is the product of each independent event.

• In a cross between pea plants that are heterozygous for flower color (Pp), what is the probability that the offspring will be homozygous recessive?

• ½ x ½ = ¼

Rule of addition

• Probability that events that can occur in 2 or more different ways will occur, is the sum of the separate problems.

• In a cross between pea plants that are heterozygous for flower color (Pp), what is the probability of the offspring being heterozygous?

• ¼ + ¼ = ½

• What is the probability that a cross between 2 organisms with genotypes AaBbCc and AaBbCc will produce an offspring with aabbcc?

• 1/4 x 1/4 x 1/4 = 1/64

Multiple Alleles

Blood Type

Genotypes Antigens in blood

Antibodies in serum

A IAIA, or

IAi

A Anti B

B IBIB, or

IBi

B Anti A

AB IAIB A, B ---

O ii --- Anti A and Anti B

Terms

• Plietropy = multiple effects from a single gene

• Epistasis = one locus alters the phenotypic expression of a second gene

• Polygenic inheritance = additive effect of 2 or more genes on a single trait

Pedigrees

• Family trees used to track human traits.

• Male =

• Female =

Genetic Screening

• Carrier recognition

• Fetal testing:– Amniocentesis– CVS

• Ultrasound

• Fetoscopy