Mendelian Genetics

Ch 14

Mendel

• Investigated variation in pea plants• Studied traits in plants• Particulate theory of inheritance– Genes maintain integrity from generation to

generation. Instead of blending together they act as discrete entities or particles.

Mendel’s laws

• Law of segregation- The two members of each gene pair

segregate or separate into different gamete cells during the formation of eggs and sperm

• Law of independent assortment– The alleles of different genes are

transmitted independently of each other

Mendel’s single factor cross

• Single factor cross, focused on one trait – i.e. flower color

• P – purebred parents, • WW x ww• F1 – all offspring were

hybrid• Ww• Monohybrid cross: • Ww x Ww• F2 – 3:1 ratio

• Looked at 2 factors,• i.e. seed shape & color• P – purebred parents• F1 – all hybrids (for both

factors)• Dihybrid cross:• RrYy x Rr Yy• Result: 9:3:3:1

Mendel’s two factor cross

1 factor cross (monohybrid) practice• Axial flowers are dominant to terminal flowers• What are the genotypic & phenotypic ratios of a

cross between a plant with heterozygous axial flowers and a plant with terminal flowers?

• 1. Choose letters to represent alleles• 2. Write genotypes of parents• 3. Write possible gametes for each parent• 4. Set up and complete punnett square• 5. Write genotypic ratio: DD: Dd: dd• 6. Write phenotypic ratio: Dominant: recessive

Test cross

• To determine unknown parent with dominant phenotype

• Cross with a recessive homozygote

• Tall dominant to short in pea plants• A cross between tall plant and short plant

produces ½ tall plants, ½ short plants. What is the genotype of the tall plant?

2 Factor cross (Dihybrid) - practice

• In rabbits Gray hair (G) is dominant to white (g), and Black eyes (B) are dominant to red (b).

• Cross a heterozygous gray, red eyed rabbit with a homozygous gray, heterozygous black eyed rabbit; what is the phenotypic ratio?– Determine gametes (use FOIL)– Set up punnett square– Determine phenotypic ratio: D/D: D/R: R/D: R/R (D =

dominant, R = recessive)

Codominance

Both alleles are expressedCRCR x CWCW – Red flower, white flowerOffspring: CRCW – red & white flower

Incomplete dominance

CRCR x CWCW – Red flower, white flowerOffspring: CRCW – hybrid is pink – inbetween two parents

Blood type

Multiple alleles: IA, IB, iShows Codominance, as well as complete dominance

• Multiple alleles – more than 2 alleles for a character

Epistasis – phenotypic expression of one gene is affected by another

Pleitropy – ability of a gene to affect an organism in many ways, i.e. sickle cell disease has multiple effects

Effects of Sickle Cell allele

Polygenic inheritance – multiple genes affect a single phenotypic character

Polygenic inheritance of skin color

Probability & Genetics

• Rule of multiplication – probability that two or more independent events occur together

• Multiply individual probabilities

• Rule of addition – probability of an event that can occur in two or more different ways –

• Add separate probabilities

• Solving genetics problems by probability:

• Hybrid Tall plants – Tt• What is the probability of each parent having

the recessive allele?• 1/2• What is the probability of the parents

producing a short plant?

• ½ x ½ = ¼

Dihybrid cross• YyRr x YyRr• Can take each trait separately• Probability of yellow offspring: • YY, Yy = 3/4• green offspring: • yy = 1/4• Probability of round offspring: • RR, Rr = 3/4• wrinkled offspring: • rr = 1/4

• Probability – • Yellow & round• ¾ x ¾ = 9/16• yellow & wrinkled• ¾ x ¼ = 3/16• green & round• ¼ x ¾ = 3/16• green & wrinkled• ¼ x ¼ = 1/16

• Probability of yellow seed• 9/16 + 3/16 = 12/16 = 3/4