HUBS1403 Lecture Notes - Lecture 16: Meiosis, Genetic Load, Sexual Reproduction
8 Jun 2018
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Basics of Hereditary
DNA Structure
• Repeating polymer of nucleotides which has 1 of 4 nitrogenous bases - A, G, T, C
• Is transcribed into mRNA which is read in triplet codons and translated into a protein
• Proteins then undertake functions within cells
DNA is packaged into chromosomes
• Molecular carrier of hereditary information
• Packaged within the nucleus into chromosomes
o Contain genes (regions that code for proteins)
Human Chromosomes
• Human somatic cells are diploid
o 23 pairs of chromosomes
• 22 pairs of autosomes + 1 pair of sex chromosomes
• Human gametes (eggs and sperm) are haploid
o 22 autosomes and 1 sex chromosome
Homologous Chromosomes
• Diploid cells have 2 copies of each chromosome
o Called homologous chromosomes
• 1 paternal and 1 maternal
• Each contain the same genes
▪ Alternative forms of a gene are called alleles (have slightly different
sequence of nucleotides)
Alleles and Gene Function
• Alleles are variants of gene
o Code for a variant of a protein
• Everyone has 2 alleles (maternal and paternal) of each gene
o Called a gene pair
o May be identical or different
• Identical alleles in a gene pair - homozygous
• Different alleles in a gene pair - heterozygous
Traits (characteristics)
• Measureable/observable feature
o Eg hair colour, height, intelligence
• Influenced by genetics (eg eye colour)
• Some ca be influenced by environment
o Eg skin colour - influenced by amount of exposure to UV light in addition to genetics
• Can be due to a single gene function or multiple genes - multifactorial
Phenotype and Genotype
Phenotype
Form that is shown (observable)
Genotype
Alleles responsible for a phenotype or train (1 from mum, 1 from dad in each gene
pair)
Dominant and Recessive Traits
Homozygous gene pairs (identical alleles)
• Only 1 option in protein function and thus display the corresponding trait
Heterozygous gene pairs ( 1 dominant and 1 recessive allele)
• Displays the dominant trait --> dominant phenotype
o Recessive allele function is masked by the function of the dominant allele
Example: Cystic Fibrosis
• Normal C or CF c
• CF disease: both recessive alleles (cc)
• No CF disease: homozygous dominant (CC) or heterozygous (Cc)
Simple vs Complex Genetics
Simple
Considers a single trait influences by a single gene with clear dominant and recessive
inheritance patterns
Eg CF, achondroplasia
Complex
Considers traits influenced by either multiple genes and/or the environment
Eg spina bifida, height
Variations of Dominance
• Co-dominance
o Both alleles in a gene pair are dominant; both are expressed in the phenotype equally
o Eg ABO blood grouping
o A = dominant
o B = dominant
o O = Recessive
o Blood type AB = heterozygous dominant
• Incomplete dominance
o Both alleles in a gene pair contribute to the phenotype giving rise to an intermediate
phenotype
o Eg sickling gene(s) - sickle cell anaemia
o S = dominant= normal blood cells, s = recessive= sickle shaped RBC
o Ss = ½ normal and ½ sickle shaped
Heredity and Variation
• Sexual reproduction (meiosis) creates genetic variation in the gametes of an individual
o First observed by Mendel in the 1800s
o Deduced 2 laws to explain patterns of traits within populations
• Segregation
• Independent assortment
Mendel's Law of Segregation
In the formation of gametes, 2 members of a gene pair (alleles) segregate into different haploid
gametes with equal probability
Mendel's Law of Independent Assortment
• During meiosis the chromosome pairs are split (law of segregation) and divided independent
of each other into gametes
• Different genes assort independently of each other
o As long as they are on different chromosomes
• Eg with the peas - height and colour are on different chromosomes so sort
independently of each other
