Patterns of Inheritance

Patterns of Inheritance

Introduction

The study of genetics is fundamental in understanding how traits and genetic disorders are passed from one generation to another. Patterns of inheritance play a significant role in genetic screening and counselling, helping healthcare professionals assess the risk of genetic disorders in individuals and families. In this section, we will explore different patterns of inheritance, including autosomal recessive, autosomal dominant, incomplete dominance, and sex-linked recessive single gene disorders, and how they are analysed in genetic screening and counselling.

Patterns of Inheritance

1. Autosomal Recessive Inheritance

• Definition: Autosomal recessive inheritance refers to the inheritance pattern of a recessive allele located on an autosomal chromosome (non-sex chromosome). In this pattern, individuals must inherit two copies of the recessive allele (homozygous) to express the associated disorder.
• Example: Cystic fibrosis is an autosomal recessive disorder. Individuals with two copies of the mutated CFTR gene (homozygous) develop the condition.

2. Autosomal Dominant Inheritance

• Definition: Autosomal dominant inheritance involves a dominant allele located on an autosomal chromosome. In this pattern, inheriting just one copy of the dominant allele (heterozygous) is sufficient to express the associated disorder.
• Example: Huntington's disease is an autosomal dominant disorder. Individuals who inherit one mutated HTT gene (heterozygous) will develop the disease.

3. Incomplete Dominance

• Definition: Incomplete dominance occurs when neither allele is completely dominant over the other. In this pattern, the heterozygous individual displays an intermediate phenotype between the two homozygous conditions.
• Example: In snapdragons, the allele for red flower color (RR) is incompletely dominant over the allele for white flower color (WW).

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Patterns of Inheritance

4. Sex-Linked Recessive Inheritance

• Definition: Sex-linked recessive inheritance involves recessive alleles located on the sex chromosomes (X or Y). In this pattern, females (XX) must inherit two copies of the recessive allele to express the disorder, while males (XY) only need to inherit one copy.
• Example: Hemophilia is a sex-linked recessive disorder. A male inheriting one mutated X-linked allele (XhY) will have hemophilia, while females need two mutated X-linked alleles (XhXh) to develop the condition.

Analysis of Family Histories

Three Generations

Genetic screening and counselling often involve the analysis of family histories spanning three generations. This allows healthcare professionals to identify patterns of inheritance and assess the risk of genetic disorders in offspring.

Standard Genetic Terms and Symbols

Genetic counselling relies on standard terms and symbols, including:

• Alleles: Different forms of a gene.
• Dominant: An allele that masks the effect of another allele.
• Recessive: An allele whose effect is masked by a dominant allele.
• Homozygous: Having two identical alleles for a particular gene.
• Heterozygous: Having two different alleles for a particular gene.
• Carriers: Individuals who carry a recessive allele for a genetic disorder but do not express the disorder themselves.
• Genotype: The genetic makeup of an individual.
• Phenotype: The physical characteristics or traits expressed by an individual.
• Autosomes: Non-sex chromosomes.
• Sex Chromosomes: The X and Y chromosomes that determine an individual's sex.

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Patterns of Inheritance

Drawing, Analysing, and Interpreting Family Histories

Genetic Pedigrees

Family histories are often represented using genetic pedigrees, which use symbols to represent family members, their relationships, and their genetic traits.

Tracking Genetic Disorders

By analysing these pedigrees, healthcare professionals can track the inheritance of genetic disorders, identify carriers, and estimate the risk of a disorder in future generations.

Summary