Intro to Genetics

p. 263 - 269

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 parentWhat are the parent alleles?

T T Homozygous

T TWhat gametes can it make?

PURE SHORT parent What are the parent alleles?

t t homozygous

t tWhat gametes can it make?

Possible Results of Offspring:GENOTYPE = Tt PHENOTYPE tall

T Tt

t

T t T t

T t T tProbabilities 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

T t

T TT

(tall)

Tt

(tall)

t Tt

(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

T t

T TT

(tall)

Tt

(tall)

t Tt

(tall)

tt

(short)