Punnet Squares BIO 121

Genetics

the study of heredity

Heredity

passing of traits from parent to offspring

Trait

an inherited characteristic such as eye color or height

Gene

a unit of DNA controlling a particular trait

Locus

the location of a gene on a chromosome

Allele

alternative form of a gene (e.g., F or f)

Dominant allele

masks the effect of a recessive allele (F)

Recessive allele

hidden when dominant allele is present (f)

Diploid (2n)

two sets of homologous chromosomes (after mitosis)

Haploid (n)

one set of chromosomes (after meiosis)

Genotype

genetic makeup (e.g., Ff)

Phenotype

observable expression of the genotype (e.g., blue eyes)

Homozygous

two identical alleles (FF or ff)

Heterozygous

two different alleles (Ff)

Who is the “Father of Genetics”?

Gregor Mendel

What organism did Mendel study?

garden pea (Pisum sativum)

What did Mendel discover?

mathematical patterns of inheritance

Particulate Theory of Inheritance

genes are reshuffled between generations but remain distinct

True-breeding

organisms that consistently pass the same trait to offspring (homozygous)

Monohybrid cross

cross involving one trait

Law of Segregation (definition)

each individual has two alleles per trait, which separate during gamete formation; fertilization restores two

Result of Mendel’s monohybrid cross

F2 ratio = 3 dominant : 1 recessive

Purpose of a Punnett square

predicts offspring genotypes and phenotypes

Test cross purpose

determine if dominant phenotype is homozygous or heterozygous

How is a test cross performed?

cross with homozygous recessive individual

Dihybrid cross

cross involving two traits

Phenotypic ratio for dihybrid cross

9 : 3 : 3 : 1

Law of Independent Assortment

allele pairs for different traits separate independently during meiosis

Sex chromosomes

X and Y (XX = female, XY = male)

Autosomes

all chromosomes other than sex chromosomes

X-linked gene

gene located on the X chromosome

Why do males show X-linked traits more often?

they have only one X chromosome, so recessive traits show up

Who studied X-linked inheritance in fruit flies?

Thomas Morgan

What trait did Morgan study?

eye color (red = dominant R, white = recessive r)

Morgan’s conclusion

the eye-color gene is on the X chromosome

Female carrier notation

XᴿXʳ

Male with trait notation

XʳY

What does “carrier” mean?

a heterozygous female who carries one recessive allele but does not show the trait

Example ratio for monohybrid genotype

1 : 2 : 1 (FF : Ff : ff)

Example ratio for monohybrid phenotype

3 : 1

When does segregation of alleles occur?

during meiosis

What restores the diploid state?

fertilization