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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

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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

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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

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• 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

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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

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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)

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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

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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

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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

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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

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