Mendelian Genetics: Sep 3

Mendelian Genetics

Introduction to Mendelian Genetics

Mendelian genetics is the study of how traits are inherited through discrete units known as genes. Gregor Mendel's experiments with pea plants in the mid-19th century laid the foundation for classical genetics, revealing predictable patterns of inheritance.

• Gregor Mendel published his experiments with pea plants in 1866, without knowledge of chromosomes or meiosis.

• He determined that units of inheritance (now called genes) exist and can predictably influence traits.

• Mendel showed that traits such as flower color are controlled by genes that occur as pairs (alleles).

Mendel's Experimental System

Mendel followed seven distinct traits in pea plants, each with two contrasting forms. He used true breeding strains to ensure consistency in his experiments.

• True breeding strains: Plants that, when self-fertilized, produce offspring with the same form of a particular trait.

• Self-fertilization: Pea plants produce both pollen (sperm) and ovules (eggs), allowing controlled crosses.

Trait	Dominant Form	Recessive Form
Flower Color	Purple	White
Flower Position	Axial	Terminal
Seed Color	Yellow	Green
Seed Shape	Round	Wrinkled
Pod Shape	Inflated	Constricted
Pod Color	Green	Yellow
Stem Length	Tall	Dwarf

Monohybrid Crosses

Monohybrid crosses involve true-breeding individuals differing in only one trait. These experiments revealed how traits are transmitted from generation to generation.

• P1: Parental generation

• F1: First filial generation (offspring of P1)

• F2: Second filial generation (offspring of F1 self-fertilization)

• Example: Crossing green seeds (recessive) with yellow seeds (dominant) yields F1 plants that are all yellow. F2 plants show a 3:1 ratio of yellow to green.

Results of Monohybrid Crosses

In Mendel's monohybrid crosses, F1 offspring were identical to one parent, but a 3:1 ratio of dominant to recessive traits was observed in the F2 generation.

Character	Dominant Trait	Recessive Trait	F2 Ratio
Flower color	Purple	White	3.15:1
Seed color	Yellow	Green	3.01:1
Seed shape	Round	Wrinkled	2.96:1
Pod shape	Inflated	Constricted	2.95:1
Pod color	Green	Yellow	2.82:1
Flower position	Axial	Terminal	3.14:1
Stem length	Tall	Dwarf	2.84:1

Mendel's Postulates

Mendel proposed three fundamental postulates to explain his results:

1. Unit factors (genes) pass unchanged from one generation to the next and determine traits. Individuals have a pair (two alleles) of each unit factor.

2. One unit factor in a pair dominates the other, which is recessive.

3. Unit factors segregate randomly during gamete formation.

Reciprocal Crosses

Reciprocal crosses involve switching the traits carried by male and female parents. Mendel found that the results were not affected by which parent carried a particular form of a trait.

• For example, yellow seed color always dominated green, regardless of which parent contributed the trait.

Alleles and Genotypes

Alleles are alternative forms of a gene. Genotype refers to the genetic makeup, while phenotype is the observable trait.

• Dominant allele: Shown in uppercase (e.g., D)

• Recessive allele: Shown in lowercase (e.g., d)

• Homozygous genotype: Two identical alleles (DD or dd)

• Heterozygous genotype: Two different alleles (Dd)

Punnett Squares

Punnett squares are used to predict the genotypes and phenotypes of offspring from genetic crosses.

• Example: DD (tall) x dd (short) yields all Dd (tall) offspring in F1.

• Self-fertilization of F1 (Dd x Dd) yields F2 with genotypic ratio 1:2:1 (DD:Dd:dd) and phenotypic ratio 3:1 (tall:short).

Punnett Square Example

	D	d
D	DD	Dd
d	Dd	dd

Genotypic ratio: 1 DD : 2 Dd : 1 dd Phenotypic ratio: 3 tall : 1 short

Test Crosses

Test crosses are used to determine the genotype of an individual expressing the dominant phenotype by crossing it with a recessive homozygote.

• If the unknown is homozygous dominant (DD), all offspring will show the dominant trait.

• If the unknown is heterozygous (Dd), half the offspring will show the dominant trait and half the recessive trait.

Dihybrid Crosses and Independent Assortment

Dihybrid crosses involve individuals differing in two traits. Mendel's fourth postulate, independent assortment, states that traits are inherited independently.

• Genotypic frequencies can be predicted using the product law of probability.

• Example: Probability of F2 plant having round, yellow seeds is $(3/4) \times (3/4) = 9/16$.

• Probability of round, green seeds is $(3/4) \times (1/4) = 3/16$.

Dihybrid Cross Punnett Square

Seed Shape	Seed Color	Phenotype
Round	Yellow	Dominant for both
Round	Green	Dominant shape, recessive color
Wrinkled	Yellow	Recessive shape, dominant color
Wrinkled	Green	Recessive for both

Product Law of Probability

The product law states that the probability of two independent events occurring together is the product of their individual probabilities.

• Example: Probability of F2 plant being short with purple flowers and green pods: $(1/4) \times (3/4) \times (3/4) = 9/64$.

Pedigree Analysis (Preview)

Pedigree analysis is used to determine the most likely mode of inheritance in families. It is a key tool for studying genetic disorders and inheritance patterns in humans.

• Pedigrees use standardized symbols to represent individuals and their relationships.

• Modes of inheritance include autosomal dominant, autosomal recessive, and X-linked patterns.

Additional info: Pedigree analysis will be covered in detail in a subsequent session.