Patterns and Processes of Inheritance: Mendelian Genetics, Extensions, and Probability

Patterns and Processes of Inheritance

Introduction to Mendelian Genetics

This section introduces the foundational principles of inheritance as discovered by Gregor Mendel, focusing on how traits are passed from one generation to the next using pea plants as a model system.

• Punnett Square: A diagram used to predict the outcome of a genetic cross by showing all possible combinations of alleles.

• Test Cross: A cross between an individual with an unknown genotype and a homozygous recessive individual to determine the unknown genotype.

• Genotype: The genetic makeup of an organism (e.g., RR, Rr, rr).

• Phenotype: The observable traits of an organism (e.g., round or wrinkled seeds).

Example: Crossing true-breeding round (RR) and wrinkled (rr) pea plants produces F1 offspring that are all heterozygous (Rr) and display the round phenotype.

Mendel's Laws of Inheritance

Mendel's laws describe how alleles segregate and assort independently during gamete formation, leading to predictable patterns of inheritance.

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation so that each gamete carries only one allele.

• Law of Independent Assortment: Genes for different traits assort independently of one another in gamete formation.

Example: A monohybrid cross (Rr x Rr) yields a 3:1 ratio of round to wrinkled seeds in the F2 generation.

Extensions to Mendelian Inheritance

Not all traits follow simple Mendelian patterns. Some traits exhibit incomplete dominance or co-dominance, where the heterozygote displays a unique phenotype.

• Incomplete Dominance: Neither allele is completely dominant; the heterozygote shows a blend of both traits (e.g., red and white flowers produce pink offspring).

• Co-dominance: Both alleles are fully expressed in the heterozygote (e.g., AB blood type in humans).

Example: In snapdragons, crossing red (RR) and white (rr) flowers produces pink (Rr) flowers in the F1 generation.

Genetic Probability and Test Crosses

Probability calculations are essential for predicting the outcomes of genetic crosses. Test crosses help determine unknown genotypes.

• Probability Formula: The probability of an event is the number of favorable outcomes divided by the total number of possible outcomes.

Example: If two heterozygous black dogs (Bb) are crossed, the probability of a yellow pup (bb) in the F2 generation is 25%.

$P(\text{yellow pup}) = \frac{1}{4}$

Genetic Disorders and Inheritance Patterns

Autosomal Recessive Inheritance

Some genetic disorders, such as Gaucher disease and PKU, follow autosomal recessive inheritance patterns. Both parents must carry the recessive allele for the disorder to appear in offspring.

• Carrier: An individual who has one copy of a recessive allele but does not show symptoms.

• Probability Calculation: For a heterozygous mother and an unaffected father, the probability that their child has Gaucher disease is 0.25.

$P(\text{affected child}) = \frac{1}{4}$

Sex-Linked and Multiple Allele Inheritance

Some traits are determined by genes located on sex chromosomes or by multiple alleles.

• Sex-Linked Traits: Traits controlled by genes on the X or Y chromosome (e.g., coat color in bears).

• Multiple Alleles: More than two alleles exist for a gene in the population (e.g., feather color in chickens).

Table: Genotype and Phenotype of Kermode Bears

Genotype of Female Parent Bear	Genotype of Male Parent Bear	Phenotype of Male Parent Bear
XBXB	XbY	White
Bb	BB	Black
XBXb	XBY	Black
XbXb	XBY	Black

Additional info: The table above is inferred from the context of bear coat color genetics, which involves autosomal and sex-linked inheritance.

Phenotypic Ratios and Genetic Crosses

Phenotypic ratios are used to describe the expected proportions of different traits in offspring from genetic crosses.

• Monohybrid Cross: A cross between two individuals involving one gene; typical F2 ratio is 3:1 for dominant to recessive phenotype.

• Dihybrid Cross: A cross involving two genes; typical F2 ratio is 9:3:3:1.

Example: Crossing two chickens with blue feathers (heterozygotes) yields a 1:2:1 ratio of black:blue:white offspring.

$\text{Phenotype ratio} = 1 \text{ black} : 2 \text{ blue} : 1 \text{ white}$

Practice Questions and Applications

Sample Questions

• Genotype and Phenotype Prediction: Tall plants are dominant to short. What are the possible genotypes and phenotypes when a heterozygous tall plant is crossed with a short plant?

• Probability Calculation: What is the probability that a child will inherit a recessive disorder if one parent is a carrier and the other is unaffected?

• Law of Segregation: Explain Mendel's law of segregation in your own words.

• Co-dominance vs. Incomplete Dominance: Distinguish between these two patterns of inheritance.

Additional info: These questions reinforce understanding of Mendelian genetics, probability, and extensions to inheritance patterns.