Intro to Genetics p

What is heredity? Tour of the basics: –Heredity = passing traits from parent to child –A zygote receives two genes for each trait – one on each homologous chromosome One from sperm, one from egg

What is a trait? Tour of the basics: –Trait = a specific characteristic Physical Behavioral Medical condition –Genes (proteins) + environment  traits Allele = different forms of a gene, often abbreviated by letters –ex: H or h

What is a trait? (continued) Tour of the basics: –How many alleles for a gene (ex: hitchhiker’s thumb) does a person inherit? TWO! (one from mom, one from dad) Homozygous = same 2 alleles – ex: HH or hh Heterozygous = 2 different alleles – ex: Hh

HOMOZYGOUS/HETEROZYGOUS When both alleles in the pair are the same, the organism is homozygous or pure. EX: TT or tt When both alleles in the pair are different, the organism is heterozygous or hybrid Ex: Tt

What is a trait? (continued) Tour of the basics: –How do the different alleles interact? Dominant = allele masks recessive allele –ex: H (straight thumb) Recessive = allele masked by dominant allele –ex: h (hitchhiker’s thumb) So… –HH  ? –Hh  ? –hh  ?

DOMINANT/RECESSIVE Dominant allele is represented by a capital letter. (usually the first letter of the trait) Recessive allele is represented by the SAME lower-case letter. EX: Tall = T Short = t

Observable traits Tour of the basics: –Earlobe attachment –Tongue rolling –Presence of dimples –Handedness –Freckles –Hair curliness –Hand clasping –Colorblindness –Widow’s peak hairline

Genotype vs. Phenotype Genotype = genetic makeup (combination of alleles inherited) –ex: HH, Hh, hh Phenotype = physical expression of the inherited trait (characteristic) –ex: straight thumb, hitchhiker’s thumb

Genetic Crosses Punnett Square - helps geneticists determine the probabilities (likelihood that a a particular event will occur) of which alleles (traits) will be passed on to offspring Maternal alleles Paternal alleles Offsprings’ genotypes

IN PEA PLANTS Tall is dominant over short TALL = T SHORT = t LET’S MAKE A CROSS! PURE TALL X PURE SHORT

PURE TALL parent What are the parent alleles? T T Homozygous   T T What gametes can it make?

PURE SHORT parent What are the parent alleles? t t homozygous   t t What gametes can it make?

Possible Results of Offspring: GENOTYPE = Tt PHENOTYPE tall TT t t T tT tT t T tT t T tT t Probabilities of Offspring: -GENOTYPES 4/4 = Tt, 100% TT, or 4:0 -PHENOTYPES 4/4 tall, 100% tall, 4:0 tall

Trait: plant height Inheritance pattern: Complete dominance T = tall allele t = short allele Cross: Two heterozygous plants Mendel’s Peas

Trait: plant height Inheritance pattern: Complete dominance T = tall allele t = short allele Cross: Two heterozygous plants ANSWER to Mendel’s Peas Tt TTT (tall) Tt (tall) tTt (tall) tt (short)

Probabilities: –Gentoypes (TT, Tt, & tt): As fraction: ¼ TT, ½ Tt, ¼ tt As percentage: 25% TT, 50% Tt, 25% tt As a ratio: 1 : 2 : 1 –Phenotypes (tall or short): As fraction: ¾ tall, ¼ short As percentage: 75% tall, 25% short As a ratio: 3 : 1 ANSWER to Mendel’s Peas Tt TTT (tall) Tt (tall) tTt (tall) tt (short)