Mendelian Genetics: Laws of Inheritance

 

Mendelian Genetics: Laws of Inheritance 

What are Mendel's Laws of Inheritance (Segregation, Independent Assortment, Dominance)?

Mendel's Laws of Inheritance are a set of principles that explain how traits are passed from parents to offspring. These laws were derived from Gregor Mendel's experiments with pea plants in the 19th century. Here's a breakdown of the three main laws:  

1. Law of Segregation  

• Paired genes: Each individual has two copies of each gene (alleles), one inherited from each parent.  
• Separation during gamete formation: During the formation of sperm and egg cells (gametes), these gene pairs separate, so each gamete carries only one allele for each trait.  
• Random fertilization: During fertilization, the sperm and egg combine randomly, resulting in a new individual with two alleles for each trait again.

In simpler terms: Imagine a pea plant has two alleles for seed color: one for yellow (Y) and one for green (y). During gamete formation, these alleles separate, so some sperm/egg cells carry Y and others carry y. When fertilization occurs, the offspring can inherit YY (yellow), Yy (yellow), or yy (green).  

2. Law of Independent Assortment

• Separate inheritance: Genes for different traits are inherited independently of each other. This means that the inheritance of one trait does not affect the inheritance of another.  
• Applies to unlinked genes: This law generally applies to genes that are located on different chromosomes or are far apart on the same chromosome.

In simpler terms: Imagine a pea plant has genes for both seed color (yellow/green) and seed shape (round/wrinkled). The inheritance of seed color (Y or y) does not influence the inheritance of seed shape (round or wrinkled). These traits are inherited independently.  

3. Law of Dominance

• Dominant vs. recessive alleles: Some alleles are dominant, meaning they mask the expression of other alleles (recessive alleles).  
• Phenotype expression: In a heterozygous individual (having one dominant and one recessive allele), the dominant trait will be expressed in the phenotype.  

What is the difference between genotype and phenotype?

Genotype and phenotype are two terms used in genetics to describe the genetic makeup and observable characteristics of an organism, respectively.  

Genotype

• Genetic makeup: The genotype refers to the specific set of genes or alleles an organism inherits from its parents.  
• Not always visible: It represents the genetic information carried by an organism, but it's not always directly visible or observable.  
• Example: In pea plants, the gene for seed color has two alleles: yellow (Y) and green (y). A plant can have the following genotypes: YY (homozygous dominant), Yy (heterozygous), or yy (homozygous recessive).  

Phenotype

• Observable characteristics: The phenotype refers to the observable physical or biochemical characteristics of an organism.  
• Influenced by genotype: It's the expression of the genotype, but it can also be influenced by environmental factors.  
• Example: In pea plants, the phenotype for seed color can be yellow or green. A plant with the genotype YY or Yy will have yellow seeds, while a plant with the genotype yy will have green seeds.  

What is the difference between homozygous and heterozygous genotypes?

Homozygous

• Identical alleles: Homozygous means that an individual has inherited two identical alleles for a particular gene, one from each parent.  
• Types:
• Homozygous dominant: Both alleles are dominant (e.g., AA).  
• Homozygous recessive: Both alleles are recessive (e.g., aa).
• Phenotype: In homozygous dominant individuals, the dominant trait is expressed. In homozygous recessive individuals, the recessive trait is expressed.  

Heterozygous

• Different alleles: Heterozygous means that an individual has inherited two different alleles for a particular gene, one from each parent.  
• Dominant allele expression: In heterozygous individuals, the dominant allele is expressed in the phenotype, while the recessive allele is masked.    

What is a dominant allele? What is a recessive allele?

Dominant and recessive are terms used to describe how different versions of a gene (alleles) are expressed in an individual.  

Dominant Allele

• Masks the other allele: A dominant allele is one that expresses its trait even when paired with a different allele (heterozygous condition).  
• Represented by a capital letter: Dominant alleles are typically represented by a capital letter (e.g., A).  
• Example: In pea plants, the allele for purple flowers (P) is dominant over the allele for white flowers (p). If a plant has one P allele and one p allele (Pp), it will have purple flowers because the P allele masks the expression of the p allele.  

Recessive Allele

• Masked by the dominant allele: A recessive allele is one that is only expressed when an individual has two copies of it (homozygous condition).  
• Represented by a lowercase letter: Recessive alleles are typically represented by a lowercase letter (e.g., a).  
• Example: In pea plants, the allele for white flowers (p) is recessive. A plant will only have white flowers if it has two p alleles (pp). If it has at least one P allele (PP or Pp), it will have purple flowers.  

 

How do dominant and recessive alleles interact to determine phenotype?

Dominant and recessive alleles interact in a way that determines which trait is expressed in an individual's phenotype. 

Homozygous Dominant (AA)

• Both alleles are dominant.  
• The dominant trait is expressed in the phenotype.
• Example: In pea plants, if both alleles are for purple flowers (PP), the plant will have purple flowers.

Homozygous Recessive (aa)

• Both alleles are recessive.
• The recessive trait is expressed in the phenotype.  
• Example: In pea plants, if both alleles are for white flowers (pp), the plant will have white flowers.

Heterozygous (Aa)

• One allele is dominant, and the other is recessive.  
• The dominant allele masks the expression of the recessive allele.  
• The dominant trait is expressed in the phenotype.  
• Example: In pea plants, if the alleles are for purple (P) and white (p) flowers (Pp), the plant will have purple flowers because the P allele is dominant over the p allele.

What is a monohybrid cross? What is a dihybrid cross? 

Monohybrid Cross

• One trait: A monohybrid cross involves the study of inheritance patterns for a single trait.  
• Example: Crossing pea plants with different flower colors (e.g., purple and white) to study how flower color is inherited.  
• Genotypes: Typically involves tracking the inheritance of two alleles for a single gene.  
• Phenotypic ratio: In a monohybrid cross between two heterozygous individuals, the expected phenotypic ratio in the offspring is 3:1 (3 dominant phenotypes to 1 recessive phenotype).

Dihybrid Cross

• Two traits: A dihybrid cross involves the study of inheritance patterns for two different traits simultaneously.  
• Example: Crossing pea plants with different seed colors (e.g., yellow and green) and seed shapes (e.g., round and wrinkled) to study how these two traits are inherited together.  
• Genotypes: Typically involves tracking the inheritance of four alleles for two different genes.
• Phenotypic ratio: In a dihybrid cross between two heterozygous individuals, the expected phenotypic ratio in the offspring is 9:3:3:1.  

How can Punnett squares be used to predict the genotypes and phenotypes of offspring from a cross?

1. Determine the genotypes of the parents:

• Identify the alleles for the trait being studied.
• Determine whether each parent is homozygous dominant, homozygous recessive, or heterozygous.  

2. Set up the Punnett square:

• Draw a grid with the number of rows and columns equal to the number of alleles each parent can contribute.
• Write the alleles of one parent across the top of the grid and the alleles of the other parent down the side.

3. Fill in the Punnett square:

• Combine the alleles from the top and side of each box to represent the possible genotypes of the offspring.  

4. Determine the genotypes and phenotypes of the offspring:

• Analyze the Punnett square to determine the possible genotypes and their frequencies.  
• Use the genotypes and knowledge of dominant and recessive alleles to determine the corresponding phenotypes and their frequencies.