Genetic Inheritance

Genetic Inheritance

1. Definitions

   • Gene- The smallest unit of heredity corresponding to section of DNA that codes for a specific mRna that is, a section of DNA located between a start codon and a stop codon
   • Chromosome- The structures that are formed within the nucleus of a cell by the super-collided DNA - 46 chromosomes (23 pairs of homologous chromosomes) 44 = autosomes 2 = sex chromosomes
   • Locus- the specific point at a chromosome where a gene is located
   • Allele -Two matched genes at same location (locus)on chromosome
   • Homozygous- If two alleles code for same trait (FF, ff)
   • Heterozygous-If two alleles different (Ff)
   • Genotype-The genetic composition of an individual (ff,FF,Ff)
   • Phenotype – The physical expression of a genotype (Brown hair)
   • Dominant - If allele masks or suppresses the expression of the other allele it is said to be dominant (represented by capital letter)
   • Recessive-The allele masked is said to be recessive (represented by lowercase letter)

2. Autosomal Dominant

   • Genes that will dominant a recessive gene (FF, Ff - phenotype will show)
     • Normal skin pigment, freckles, ability to roll tongue. normal arches, dimples, unttached earlobs (FF,Ff)

3. Autosomal Recessive

   • Gene that will only show with two homozygous recessive gene
     • Albinsm, No freckles, no tongue roll,no dimples, flat feet, attached earlobes (ff)

4. Sex-Linked Inheritance

   • Inherited traits determined by genes on sex chromosome • A gene found only on X-chromosome = X-linked • A gene found only on Y-chromosome = Y-linked • If recessive not masked – so will be expressed in males
     • If recessive not masked – so will be expressed in males - Color blind,haemophilia

5. Multiple Allele Inheritance

   • Some genes exhibit more than two allele forms e.g. blood group
     • Ia, Ib are co-dominant; i is recessive
       • You can be blood group A, B, O or AB

6. Incomplete Dominance

   • Both alleles expressed in heterozygous to produce a different phenotype to either homozygous trait, often an intermediate
     • inheritance of hair texture, flower colours – CC is curly hair cc is straight hair – Cc is wavy hair

7. Polygenic Inheritance

   • Some traits require several gene pairs
     • Skin colour based on three gene pairs, each with 2 alleles Phenotype is continuous from very fair to very dark

8. Genetic Diseases

   • Cystic Fibrosis – autosomal recessive • Sickle cell anaemia- co-dominance • Haemophilia- X-linked recessive • Down syndrome - Trisomy 21 – chromosomal abnormality