Genetics Study of heredity – Passing of traits from parent to offspring Traits – Inherited characteristics through sexual reproduction – Characteristics are inherited in pairs Alleles – different versions of the same gene are provided from both father (paternal) and mother (maternal) during egg fertilization We have 2 alleles for every gene – eye colour, hair colour ~ 21,000 genes in the human genome

Dominant and Recessive Alleles Dominant (D) – Allele that is expressed – Indicated by capital letter Recessive (d) – Allele that is hidden or suppressed if dominant allele is present – Indicated by lower case letter

Dominant vs Recessive Law of Dominance – When two organisms, each homozygous for two opposing traits are crossed (BB vs bb), offspring will be hybrid – Two different alleles are carried by individual (homologous chromosomes) – Dominant trait will on be shown – Recessive trait will remain hidden – Offspring of parents will be hybrid (Bb) Dominant traits – expressed as either BB or Bb Recessive traits – Expressed as only bb

Genotype Specific combination of alleles we have for a trait – Homozygous dominant – BB – Homozygous recessive – bb – Heterozygous - Bb

Probability Important in genetics because we can predict outcomes (what is the chance that something will occur) A Punnett Square is used to determine outcomes Outcomes are genetic traits of an individual

Multiplication and Addition Rule Multiplication Rule – When two independent events are happening – Multiply the chance of one event happening by the chance that the other will happen – Ex: chances of having two boys – ½ x ½ = ¼ Addition Rule – More than one outcome of a specific event – Add the probabilities of each of those outcomes – Ex: chances of having a boy and a girl – ½ x ½ = ¼ of having a boy – ½ x ½ = ¼ of having a girl – ¼ + ¼ = ½ of having a girl and a boy

The Law of Segregation During meiosis (gamete formation) homologous gene pairs separate Number of chromosomes reduced by half Two traits carried by each parent separate Focuses on a single gene

What are Gametes Formation of sperm (4) and egg (1) – Spermatogenesis and oogenesis Each sperm will carry an allele of a specific gene One egg will carry an allele of a specific gene

Monohybrid Crosses Breeding between a male and a female to produce offspring (fertilization of sperm and egg) Punnett square – Used to predict inheritance of a trait in individuals Monohybrid cross – Genetic cross between parents who are hybrid for one trait – Ex: Colour of the bird – Phenotype is 3:1 – Genotype is 1:2:1

Phenotype Physical description of an organisms trait Related to the genotype of a gene (BB, Bb, bb)

Test Cross Back Cross Determine the genotype of an individual who only shows the dominant trait Individual can either be homozygous or heterozygous for a specific gene Breeding occurs between a phenotypically dominant individual and a phenotypically recessive individual

Test Cross Example BB bBb b Bb b b bb B = black b = white If the parent of the unknown genotype is BB, there can be no white offspring B = black b = white If the parent of the unknown genotype is hybrid, there is a 50% chance that any offspring will be white

Law of Independent Assortment (Dihybrid Cross) Alleles of the same gene (Aa) will separate during meiosis and segregate independently from alleles of another gene (Bb) Assortment of genes (chromosomes) is completely random

Dihybrid Cross A cross between individuals who are homozygous for different traits will produce offspring (F₁ generation) who are dihybrid for that specific trait (TtYy) Resulting genotypes for F₂ generation will show a 9:3:3:1 relationship ALWAYS

Beyond Mendelian Laws Applies to traits determined by a single gene (two alleles) Does not explain situations involving two or more genes Variety of phenotypes Incomplete dominance, Codominance, Multiple alleles, Pleiotropy, Epistasis, Polygenic inheritance

Incomplete Domiance Characterized by a blending of genes in heterozygous individual Neither trait is domiant All capital letters are used to write the genes A all red snapdragon (RR) is crossed with a white snapdragon (WW) producing all pink snapdragon’s (RW) If two pink snapdragon’s are crossed; – 25% chance offspring is red, 25% white, and 50% pink

Codominance Both traits are shown in heterozygous individual

Multiple Alleles Multiple forms (alleles) of a gene Human blood types are encoded by a single gene with three alleles I A, I B, and I O I A and I B code for two different protein, cell surface antigen A or antigen B I O codes for absence of cell surface antigen

Pleiotropy One single gene possessing multiple traits which can affect an organism in a variety of ways PKU – inability to breakdown phenylalanine Sickle-cell anemia – mutation in hemaglobin causing irregular shape in RBC Cystic fibrosis – abnormal thickening of mucus that covers cells

Epistasis Two separate genes control one trait, but ONE gene will mask the expression of the other gene The gene that masks the expression of the other gene is known as epistatic

Polygenic Inheritance Results from a blending of several separate genes Two short parents carry more genes for shortness than for tallness, however, their child can inherit mostly tall genes Genotypes always result in bell-shaped curve Ex; skin pigmentation

Linked Genes If two genes are located on the same chromsome, they will not assort independently Unless separated by cross over event ~21,000 genes and only 46 chromosomes 46 linkage groups

Sex Linked Traits Traits located on the X chromosome (X-linked) – Genes on the X chromosome (X N ) Few genes located on the Y chromosome (Males) Recessive Mutation – X N X N – Females carrying only one gene are known as carriers Dominant Mutation – X N X Males affected by recessive x- linked traits more than females – Hemophilia, colour blindness, Duchenne muscular dystrophy

Factors Affecting Sex – Linked Traits All daughters of affected fathers are carriers Sons cannot inherit a sex-linked trait from the father (sons inherit Y chromosome) Son has a 50% chance of inheriting a sex linked trait from a carrier mother There is no carrier state for X-linked traits in males A male will express the gene is he has it Uncommon for females to have a recessive sex-linked condition (must inherit mutated genes from both parents) X-X- Y XX-XX-XXY XX-XX-X XY X-X- X-XX-XX-YX-Y XX

Pedigree Visual representation of how a trait is inherited in different generations Females represented by a circle and males a square Carrier state is not always shown (sometimes half shaded circle or square) Person who exhibits the trait is completely shaded

X Inactivation – Barr Body ONE X chromosome in every female somatic cell (body) is inactivated Occurs randomly (some cells have on X inactivated, some cells have the other) Inactivated chromosome condenses into a dark spot Observed at outer edge of the nucleus

X Inactivation – Barr Body – Calico Cats Alleles for black and yellow fur carried on X chromosome Males can be X Y Y (yellow) or cY (black) Calico cats have patches because of Barr bodies (X Y X B ) Fur develops from cells with different deactivated X chromosomes