Human Genetics Unit

Creating Genetic Diversity How did you become so unique? 1) Sexual reproduction 2) Independent assortment of chromosomes (from mom and dad) 3) Random fertilization of gametes

Lets think about this Egg and Sperm with 223 (or about 8 million) different combinations of chromosomes are produced through meiosis Sexual reproduction involves egg and sperm A sperm (with 223 different chromosome combinations) can fertilize an egg (with 223 different chromosome combinations) Any sperm can fertilize any egg

Lets think about this So what does all that mean????????? You are the product of 223 x 223 (or more than 70 trillion) different combinations of chromosomes Or You are the outcome of 1 in 70 trillion (70,000,000,000,000) different combinations of chromosomes from mom and dad

Creating Genetic Diversity (cont) 4) Crossing over = the exchange of chromosome segments between homologous chromosomes - occurs during meiosis

1) Two homologous chromosomes pair up with each other during prophase I of meiosis. 2) In this position, some chromatids are very close to each other and segments cross. 3) Some of the segments break off and reattach to other chromosomes

= genes located close together tend to be inherited together Gene Linkage = genes located close together tend to be inherited together - discovered by American scientist Thomas Hunt Morgan - studied fruit flies

Genetic Linkage Crossing over is more likely to happen between B and C, separating A &B from C & D.

- Morgan’s Conclusions 1) Linked genes are on the same chromosomes Gene Linkage (cont) - Morgan’s Conclusions 1) Linked genes are on the same chromosomes 2) The farther apart two genes are located, the more likely they are to be separated by crossing over (and vice versa)

Genetic Linkage Crossing over is more likely to happen between B and C, separating A &B from C & D.

- linkage maps show relative location of genes on a chromosome Gene Linkage (cont) - linkage maps show relative location of genes on a chromosome

Polygenic Traits = traits produced by two or more genes - will show a range of possibilities (ex: skin color, eye color)

Polygenic Traits (cont) - epistasis = when one gene interferes with the expression of other genes - ex: albinism

Other Albino Animals

Environment Influence on Phenotype - Are identical twins always IDENTICAL? No - nutrition, health care, money, disease, injuries

Environmental Influence on Phenotype (cont) In animals: - Light – controls the color expression in some insects

Environmental Influence on Phenotype (cont) In animals: Temperature controls the sex of certain types of reptiles

Environmental Influence on Phenotype (cont) In animals: Temperature controls coloration in some mammals

Environmental Influence on Phenotype (cont) External alteration of phenotype does not affect genotype your phenotype is a mixture of your genes (genotype) and the environment you live in

Sex Determination - Male = XY Female = XX - normally, 50% of the offspring of an organism born are male, 50% are female

Sex Determination (cont) Y Genotypic Ratio Phenotypic Ratio 2 XY: 2 XX XX X XY XX XY X 2 Males: 2 Females

= traits caused by genes located on the sex chromosomes Sex-Linked Traits = traits caused by genes located on the sex chromosomes - most genes that determine your phenotype are found on autosomes, however, some are found on sex chromosomes

Sex-Linked Traits (cont) - most sex-linked traits are found on the X chromosome - males are more likely to show a sex-linked trait or disorder than females - Why? Because males only have one X chromosomes

Sex-Linked Traits (cont) - Example 1: Red eyes are dominant to white eyes in fruit flies. Cross a homozygous dominant red-eyed female with a red-eyed male. Eye color is a X-linked trait.

Example 1 X Y X X X X Y R R R R R R R R R

Genotypic Ratio 2 XRXR: 2 XRY Phenotypic Ratio 2 red-eyed females: 2 red-eyed males

Sex-Linked Traits (cont) - Example 2: Red eyes are dominant to white eyes in fruit flies. Cross a heterozygous female with a red eyed male. Eye color is an X-linked trait.

Example 2 X Y X X X X Y R R R R R r R r r

Genotypic Ratio 1 XRXR: 1 XRXr: 1 XRY: 1 XrY Phenotypic Ratio 2 red-eyed females: 1 red-eyed male: 1 white-eyed male

Sex-Linked Traits (cont) - carrier = does not show a disease/disorder but can pass it onto their offspring - aka: heterozygous

Genetic Disorders 1) Single allele disorders = controlled by a single allele (letter) of a gene, either dominant or recessive - genetic marker = a short section of DNA that is known to have a close association with a particular gene located nearby

Cystic Fibrosis - Recessive

Huntington’s Disease - Dominant

Genetic Disorders (cont) 2) monosomy = a zygote with 45 chromosomes - has only one copy of a particular chromosome

Genetic Disorders (cont) 3) trisomy = a zygote with 47 chromosomes - has three copies of a particular chromosome - most common example is down syndrome (aka: trisomy 21) - has an extra copy of chromosome 21

Genetic Disorders with the Sex Chromosomes - XO = Turner’s Syndrome - XXY = Klinefelter Syndrome - XXX = Triple X Females - XYY = Jacob Syndrome - YO = Does not exist…need an X chromosome to live What about hermaphrodites?