Lecture G3 (Mendelian Genetics)

What is a gene?

• A segment of DNA that occupies a specific place on a chromosome and controls a heritable trait

• The basic unit of heredity

What is an allele?

• An alternative version of a gene that produces different traits

• For example, purple versus white flower color

What do different alleles cause?

Variation in inherited characters

What are Mendel’s two laws of inheritance?

• Law of Segregation

• Law of Independent Assortment

What does the Law of Segregation state?

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

When do the two alleles come back together?

At fertilization, when gametes fuse to form offspring

What does the Law of Independent Assortment state?

Alleles of different genes separate into gametes independently of one another

When does independent assortment apply?

To genes on different chromosomes or far apart on the same chromosome

What phenotypic ratio did Mendel find in a dihybrid cross?

9:3:3:1

Genotype

The genetic makeup — the alleles an organism has (like PP, Pp, or pp)

Phenotype

The physical, observable traits (like purple or white flowers)

How are genotype and phenotype related?

The genotype determines the phenotype

Homozygous

Having two identical alleles for a gene

• DD or dd

Heterozygous

Having two different alleles for a gene

• Dd

Which genotype shows the recessive trait?

Homozygous recessive (dd)

Which allele determines appearance in heterozygotes?

The dominant allele

P generation

The true-breeding (pure-breeding) parent plants

F1 generation

Hybrid offspring from crossing the P generation

F2 generation

Offspring from self-pollination or cross-pollination of F1 plants

What ratio did Mendel find in F2 for a single trait?

• phenotypic ratio → 3:1 (dominant: recessive)

• Genotypic ratio → 1 PP:2Pp:1pp

What does a punnett square show?

All possible combinations of alleles in offspring from parents with known genotypes

What is the main purpose of a punnett square?

To predict genetic and phenotypic outcomes of a cross

What are gametes from a heterozygote (Pp)?

50% P and 50% p

What is a testcross?

A cross between an individual with an unknown genotype and one that is homozygous recessive

Why do a testcross?

To find out if the individual with the dominant phenotype is homozygous dominant or heterozygous

What testcross result shows a heterozygous parent?

Offspring with both dominant and recessive phenotypes (1:1 ratio)

What is a character?

A heritable feature that varies among individuals

• Flower color, seed shape

What is a trait?

A specific form of a character

• Purple or white flowers

What does “true-breeding” mean?

Always producing the same trait over many generations of self-pollination

What is hybridization?

Mating two true-breeding varieties with different traits