Oct 27 Lecture

Gametes

Mendel's Principles Recap

• Gametes: Reproductive cells that carry genetic information.

• Alleles: Different versions of a gene. Inheritance involves dominant and recessive alleles.

  • Dominant Allele: Represented by the uppercase letter (e.g., R for seed shape).

  • Recessive Allele: Represented by the lowercase letter (e.g., r).

• Chromosomes replicate during cell division before meiosis.

Meiosis

• The process through which gametes are formed; involves two stages: Meiosis I and Meiosis II.

  • Meiosis I: Chromosome pairs segregate.

  • Meiosis II: Sister chromatids segregate.

Principle of Segregation

• Definition: Pairs of alleles are separated during meiosis I in the formation of gametes.

• Example:

  • Possible arrangements during metaphase:

  • Chromosomes could line up as follows:

    • 1st Arrangement: (RR, rr)

    • 2nd Arrangement: (R,r)

Gamete Formation Example

• If both parents are heterozygous for seed shape (Rr), then the combinations of gametes from these parents can lead to:

  • ( rac{1}{4} RY, rac{1}{4} ry)

  • ( rac{1}{4} Ry, rac{1}{4} rY)

Principle of Independent Assortment

• Definition: Alleles for different traits assort independently of one another during gamete formation.

• Reasoning:

  • Genes for different traits are located on different chromosomes.

  • Each chromosome has equal chance of aligning in two ways during metaphase.

• Hypothesis: Alleles of different genes do not remain together when gametes form.

Mendel's Experiments

Traits in Pea Plants

• Traits studied include:

  • Flower Color:

  • Dominant allele (A): Purple flowers

  • Recessive allele (a): White flowers

  • Plant Height:

  • Dominant allele (T): Tall

  • Recessive allele (t): Short

• True-breeding Plants: Plants that produce offspring of the same phenotype.

Example Cross

• Crossing of True-Breeding Plants:

  • a. Short with Purple Flowers: genotype would be aaTT

  • b. Tall with White Flowers: genotype would be AATT

iClicker Questions

• Question: What is the genotype for a true-breeding short plant with purple flowers?

  • Options:

  • AATT

  • AaTt

  • aatt

  • aaTT

  • AAtt

• Question: What is the genotype for a true-breeding tall plant with white flowers?

  • Options similar to above

Dihybrid Crosses

F1 Generation Genotypes

• From the Cross:

  • Genotype of all F1 generation plants would be AaTt

• Gamete Formation for F1 Generation: Possible gametes are combinations of the alleles, such as:

  • For Aatt:

  • Producing gametes: At, at

Dihybrid Cross Overview

• Bifurcation of physical traits leads to varied phenotype expressions in progeny.

Advanced Genetic Concepts

Codominance

• Definition: A situation where two different alleles are expressed together in heterozygous individuals.

• Blood Types:

  • Example:

  • Genotype examples:

    • IAIA, IBIB, IAIB (Type AB)

• Representation: Glycoproteins on red blood cells determine blood type.

Incomplete Dominance

• Definition: A situation where heterozygotes display an intermediate phenotype, different from either homozygote.

• Example:

  • Flower Color:

  • RR (Red) x rr (White) gives rise to pink flowers in the (Rr) genotype in F1.

Summary of Principles

Mendel's Principles

• The principle of independent assortment arises because traits are unlinked and assort independently during gamete formation.

• Linked Genes: Traits that are on the same chromosome and do not assort independently.

Human Inheritance Patterns

• Geneticists' Methods: Use existing mating data to deduce inheritance patterns through pedigrees:

  • Squares indicate males

  • Circles indicate females

  • Filled symbols represent individuals displaying the phenotype of interest, while empty symbols represent those that do not

Application of Pedigrees

• Pedigrees provide insights into inheritance patterns in families and help illuminate autosomal dominant, autosomal recessive, or X-linked inheritance.