Genetics and Inheritance

Genes and Alleles

Genetics is the study of heredity and variation in living organisms. Genes and alleles are fundamental concepts in understanding how traits are passed from one generation to the next.

• Gene: A segment of DNA that contains the instructions for producing a specific protein.

• Allele: Different forms of a gene found at the same locus on homologous chromosomes.

• Locus: The specific physical location of a gene on a chromosome.

• Homozygous: Having two identical alleles for a particular gene (e.g., AA or aa).

• Heterozygous: Having two different alleles for a particular gene (e.g., Aa).

• Genotype: The genetic makeup of an organism (the combination of alleles).

• Phenotype: The observable physical or physiological traits of an organism, determined by its genotype and environment.

Example: The gene for hair color may have alleles for brown or blonde hair.

Patterns of Inheritance

Overview of Inheritance Patterns

Traits can be inherited in several different ways, depending on the nature of the gene and its alleles.

• Dominant-Recessive Inheritance

• Multiple Allele Inheritance

• Codominance

• Incomplete Dominance

• Sex-Linked Inheritance

• Polygenic Inheritance

Dominant–Recessive Inheritance

In this pattern, one allele (dominant) can mask the expression of another (recessive).

• Dominant allele: Indicated by a capital letter (e.g., A).

• Recessive allele: Indicated by a lowercase letter (e.g., a).

• For a recessive trait to be expressed, both alleles must be recessive (aa).

• Examples: Earlobe attachment, widow's peak, dimples.

Punnett squares are used to determine possible allele combinations and probabilities.

Punnett Square Example

Punnett squares are diagrams used to predict the outcome of a particular genetic cross.

Additional info: This table shows the possible genotypes for offspring from two heterozygous parents (Aa x Aa).

Multiple Alleles

Some genes have more than two allelic forms. The classic example is the ABO blood group system.

• Three alleles: IA, IB, i

• IA and IB are codominant; i is recessive.

Codominance

In codominance, both alleles are equally and fully expressed in the phenotype of the heterozygote.

• Neither allele is recessive.

• Example: Blood type AB (IAIB), where both A and B antigens are present.

Incomplete Dominance

Incomplete dominance occurs when the heterozygote has a phenotype that is intermediate between the two homozygotes.

• Neither allele is completely dominant over the other.

• Example: In snapdragons, crossing a red flower (RR) with a white flower (WW) produces pink flowers (RW).

• In humans: Sickle cell trait (heterozygous for sickle cell gene) produces both normal and sickled hemoglobin.

Sex-Linked Inheritance

Basics of Sex-Linked Inheritance

Sex-linked traits are associated with genes located on the sex chromosomes (X and Y).

• The X chromosome carries many more genes than the Y chromosome.

• Most sex-linked traits are X-linked.

• Y chromosome is much smaller and mainly determines male sex characteristics.

Human Sex Chromosomes and Variations

• Normal female: XX; normal male: XY.

• Variations include Turner syndrome (XO), Klinefelter syndrome (XXY), and others (XXX, XYY, etc.).

• These variations can result in differences in sexual development and fertility.

Sex-Linked Disorders

• Androgen Insensitivity Syndrome (AIS): A person is genetically male (XY) but resistant to male hormones, resulting in female physical traits.

• Congenital Adrenal Hyperplasia (CAH): An autosomal recessive disorder causing excess androgen production, leading to masculinization of female genitalia.

Inheritance Patterns of Sex-Linked Traits

• Females have two X chromosomes; males have one X and one Y.

• A female carrier (heterozygous) for an X-linked recessive trait can pass the allele to her sons (who will express the trait if they inherit the X with the mutation).

• Males are more likely to express X-linked recessive disorders (e.g., color blindness, hemophilia).

Example: Color Blindness

Additional info: XN = normal vision allele; Xn = color blindness allele.

Polygenic Inheritance

Definition and Examples

Polygenic inheritance occurs when multiple genes contribute to a single phenotypic trait, resulting in continuous variation.

• Traits such as skin color, height, and eye color are polygenic.

• Each gene may have two or more alleles, and their effects are additive.

• Polygenic traits often show a bell-shaped distribution in the population.

Mutation

Definition and Effects

A mutation is a permanent change in the DNA sequence of a gene. Mutations can be inherited or acquired and may have various effects on the organism.

• Mutations can cause genetic disorders (e.g., sickle cell anemia, cystic fibrosis).

• Types of mutations include point mutations, insertions, deletions, and chromosomal rearrangements.

• Some mutations are beneficial, while others are harmful or neutral.

Example: Polydactyly is a condition where an individual has extra fingers or toes, often caused by a mutation in a single gene.