Genetics: Mendelian inheritance — KCSE Biology

Punnett squares are a useful tool for predicting the genotypes and phenotypes of offspring from a monohybrid cross. A monohybrid cross involves one trait, represented by two alleles. Here’s how to use a Punnett square:

1. Identify the alleles: For example, let’s consider a trait for flower color where 'R' (red) is dominant and 'r' (white) is recessive.

2. Determine the parents' genotypes: Suppose we cross a homozygous red flower (RR) with a homozygous white flower (rr).

3. Set up the Punnett square: Place one parent's alleles across the top and the other parent's alleles along the side.

   | R | R | |-------|-------| | r | Rr | Rr | | r | Rr | Rr |

4. Fill in the squares: Combine the alleles from the top and side.

5. Analyze the results: All offspring (100%) will be heterozygous (Rr) and will display the dominant phenotype (red flowers).

Using Punnett squares helps visualize genetic crosses and predict outcomes effectively.

Sex determination in humans is primarily governed by the presence of sex chromosomes. Males have one X and one Y chromosome (XY), while females have two X chromosomes (XX). During fertilization, the sperm contributes either an X or a Y chromosome, determining the sex of the offspring.

Sex-linked traits are those that are associated with genes located on the sex chromosomes, particularly the X chromosome. Because males have only one X chromosome, any recessive trait on the X will be expressed. In contrast, females can be carriers of recessive traits without expressing them if their second X chromosome carries the dominant allele.

Examples of sex-linked traits include color blindness and hemophilia, which are more common in males.

Understanding these concepts is essential for predicting inheritance patterns in families and recognizing how certain traits can be passed down through generations.

Mendelian inheritance refers to the patterns of inheritance discovered by Gregor Mendel through his experiments with pea plants. Key concepts include:

• Genes and Alleles: A gene is a unit of heredity, while alleles are different versions of a gene.
• Dominant and Recessive Traits: Dominant alleles mask the expression of recessive alleles. For example, in pea plants, the allele for tall plants (T) is dominant over the allele for short plants (t).
• Genotype and Phenotype: The genotype is the genetic makeup (e.g., TT, Tt, tt), while the phenotype is the observable trait (e.g., tall or short).

Mendel's experiments led to the formulation of the Law of Segregation, which states that allele pairs separate during gamete formation, ensuring that offspring receive one allele from each parent.

To illustrate, consider a cross between a homozygous tall plant (TT) and a homozygous short plant (tt):

• F1 Generation: All offspring (Tt) will be tall, displaying the dominant trait.
• F2 Generation: When F1 plants are crossed (Tt x Tt), the expected phenotypic ratio is 3 tall: 1 short.

Understanding these principles helps in predicting inheritance patterns in various organisms.