AP Biology Unit 5 – Heredity: Ultimate Study Notes

What is a gene?

A segment of DNA that codes for a trait

A type of cell

A form of a gene

Homozygous vs Heterozygous

Homozygous = two identical alleles; Heterozygous = two different alleles

Dominant vs Recessive allele

Dominant masks recessive; recessive is masked

Haploid vs Diploid

Haploid = one set of chromosomes (gametes); Diploid = two sets (somatic cells)

What organism did Mendel use to study inheritance?

Pea plants

Mendel discovered that traits are inherited according to:

Specific laws (segregation and independent assortment)

Law of Segregation

Two alleles separate during gamete formation; each gamete gets one allele

Law of Independent Assortment

Genes for different traits separate independently if on different chromosomes or far apart

Testcross purpose

Determine unknown genotype by crossing with homozygous recessive

Monohybrid cross genotype ratio for Aa × Aa

1:2:1 (AA:Aa:aa)

Monohybrid cross phenotype ratio for Aa × Aa

3:1 (dominant:recessive)

Dihybrid cross phenotype ratio for AaBb × AaBb

9:3:3:1

Probability rules – AND vs OR

AND = multiply; OR = add

Pedigree basics

Squares = males, circles = females; shaded = affected

Incomplete dominance

Heterozygote shows blended phenotype

Codominance

Both alleles fully expressed

Multiple alleles

More than two alleles possible in population

Polygenic inheritance

Trait controlled by multiple genes → continuous variation

Environmental effects on phenotype

Environment can influence trait expression

Chromosomal Theory of Inheritance

Genes are located on chromosomes, explaining Mendel’s laws physically

Thomas Hunt Morgan’s discovery

Sex-linked traits in fruit flies

Linked genes

Genes close together on same chromosome tend to be inherited together

Crossing over

Occurs in prophase I → increases genetic variation

Recombination frequency formula

(# recombinant ÷ total offspring) × 100

X-linked traits

Found on X chromosome; males more affected

Carrier female

Heterozygous female who may pass trait to sons without expressing it

X-linked cross example

50%

Y-linked traits

Passed from father to son only

Sex-influenced trait

More easily expressed in one sex

Sex-limited trait

Occurs in only one sex

Law of Segregation corresponds to which meiosis event?

Separation of homologous chromosomes in anaphase I

Independent assortment mechanism

Random alignment of chromosomes at metaphase I

How does genetic variation arise?

Crossing over + independent assortment + random fertilization