26/01/2018 Genetics and Inheritance Dr Michelle Thunders [email protected] Key terms • Gene • Chromosome • Locus • Allele • Homozygous • Heterozygous • Genotype • Phenotype • Dominant • Recessive Human Cells Nuclei contain 46 chromosomes (23 pairs of homologous chromosomes) -- except gametes 44 = autosomes 2 = sex chromosomes XX= female guide expression of traits determineXY = male genetic sex 1 26/01/2018 Human chromosomes • number and size of chromosomes in a cell is the karyotype. • All somatic cells of an organism have the same karyotype. •Group A: Longest chromosomes with centromeres near middle (1,2,3) •Group B: Long chromosomes with centromeres toward one end. (4,5) •Group C: Medium sized chromosoems, meta- to submetacentric (6,7,8,9,10,11,12) •Group D: Moderately short, centromere to one end (an acrocentric may have a very short arm) (13,14,15) •Group E: Moderately short, metacentric to submetacentric (16,17,18) •Group F: Very short, metacentric (19,20) •Group G: Very short, acrocentric (21,22) •X: like the largest in group C •Y: very short, like a G group chromosome Complete Karyotype diploid onegenome from = eggtwo andsets ofone genetic from instructions sperm Marieb et al, 2007 2 26/01/2018 Gene Alleles • Chromosomes paired -- so genes paired (one from each parent) • Two matched genes at same location (locus)on chromosome = allele • Allele code for same or different form of trait • If two allele code for same trait = homozygous • If two allele different = heterozygous Alleles Example: Dimples determined by a dominant gene. • If alleleSo masks or suppresses the expression of theD = otherdominant allele dimplesit is said togene be dominant d = recessive(represented gene by capital(no dimples) letter) homozygous heterozygous • The alleleDD= masked dimples is said Ddto be= recessivedimples (represented by lowercase letter) dd =homozygous no dimples 3 26/01/2018 • Persons genetic make-up = genotype • The way genotype is expressed = phenotype Example: Dimples Genotype: DD; Dd; dd Phenotype: dimples or no dimples! Example:Punnett Dimples Square Heterozygous for dimples mother Homozygous for dimples D d D DDOffspringPossible can Dd genotypesonly have for offspring Dd Father D DDdimples!! Types of Inheritance • Autosomal Dominant /Recessive • Sex- linked Inheritance • Multiple Allele • Incomplete Dominance • Polygenic 4 26/01/2018 Dominant and Recessive Inheritance Write down your phenotype and genotype for each of these traits, others include PTC tasting (rr non-taster), ACTN athletic phenotype XX endurance, RR sprint/gymnast? Example: Albinism Autosomal Recessive Mosby, 2006 Sex-Linked Inheritance • Inherited traits determined by genes on sex chromosome sex linked • 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 e.g. colour blindness; haemophilia 5 26/01/2018 Example: Colour blindness Mosby, 2006 Protanopia (red-green CB) Deuteranopia (red-green CB) Humans – 3 opsin genes Opsin protein variations change absorption spectrum of light and thus determine range of colours Tritanopia (blue-yellow CB) 6 26/01/2018 Protanopia (red-green CB) Deuteranopia (red-green CB) http://www.the- scientist.com//?articles.view/articleNo/41055/title/The -Rainbow-Connection/ Tritanopia (blue-yellow CB) Multiple allele inheritance • Some genes exhibit more than two allele forms e.g. blood group • Three alleles: IA, IB, and i • IA, IB are co-dominant; i is recessive • You can be blood group A, B, O or AB 7 26/01/2018 Incomplete Dominance • Both alleles expressed in heterozygous to produce different phenotype to either homozygous, often an intermediate • E.g. inheritance of hair texture, flower colours – CC is curly hair – cc is straight hair – Cc is wavy hair Polygenic Inheritance • Some traits require several gene pairs e.g. skin colour and eye colour • Skin colour based on three gene pairs, each with 2 alleles Phenotype is continuous from very fair to very dark Polygenic inheritance • Eye colour, OCA2 gene 8 26/01/2018 Genetic diseases • Cystic Fibrosis – autosomal recessive • Sickle cell anaemia- co-dominance • Haemophilia- X-linked recessive • Down syndrome- Trisomy 21 – chromosomal abnormality How genes work ON/OFF GENE SWITCH How genes work ON/OFF GENE SWITCH 9 26/01/2018 Genetic variation How epigenetics works Epigenetics • Phenotype = genotype + lifestyle/environment • Overlap = epigenetics • Study of environmental factors and their impact on human health through change in gene expression 10.