gen bio 2 alleles

What is a gene?

A segment of DNA that encodes a trait

What is an allele?

A variant form of a gene, found at the same locus on homologous chromosomes

How many alleles does an individual inherit per gene, and from whom?

Two — one from each parent

Eye color allele example

B = brown (dominant), b = blue (recessive)

Flower color allele example

P = purple (dominant), p = white (recessive)

Key takeaway about alleles

Alleles are different versions of the same gene, occupying the same locus on paired chromosomes

What does a dominant allele do?

It expresses its trait even when only one copy is present

What does a recessive allele do?

It is masked unless two copies are inherited

What is genotype?

The genetic makeup of an organism (e.g., AA, Aa, aa)

What is phenotype?

The observable trait of an organism

Homozygous Dominant genotype & phenotype

AA — dominant trait expressed

Heterozygous genotype & phenotype

Aa — dominant trait expressed

Homozygous Recessive genotype & phenotype

aa — recessive trait expressed

Key takeaway about dominant/recessive alleles

A dominant allele masks the recessive allele in heterozygous individuals (Aa)

What happens during meiosis to alleles?

Homologous chromosomes separate so each gamete carries only one allele per gene

Which parent contributes which gamete?

The mother contributes one allele via the egg; the father contributes one via the sperm

What restores the two-allele state in offspring?

Fertilization

Mendel's Law of Segregation

The two alleles for a gene separate during gamete formation, so each gamete receives only one

In an Aa × Aa cross, what % of offspring are AA?

25% (homozygous dominant)

In an Aa × Aa cross, what % of offspring are Aa?

50% (heterozygous)

In an Aa × Aa cross, what % of offspring are aa?

25% (homozygous recessive)

Genotype ratio for Aa × Aa cross

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

Phenotype ratio for Aa × Aa cross

3 : 1 (dominant : recessive)

Key takeaway from the Punnett square example

Even when both parents show the dominant trait, 25% of offspring can express the recessive trait

Earlobes: dominant vs recessive trait

Detached (dominant) vs. attached (recessive)

Widow's peak: dominant vs recessive trait

Widow's peak (dominant) vs. straight hairline (recessive)

Pea seed color: dominant vs recessive trait

Yellow (dominant) vs. green (recessive)

Flower color: dominant vs recessive trait

Purple (dominant) vs. white (recessive)

Blood type inheritance example

ABO system — controlled by multiple alleles (IA, IB, i)