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
4) Crossing over
= the exchange of chromosome segments between homologous chromosomes
- occurs during meiosis
Gene Linkage
= genes located close together tend to be inherited together
- discovered by American scientist Thomas Hunt Morgan
- studied fruit flies
- 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)
- 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)
- epistasis = when one gene interferes with the expression of other genes
- ex: albinism
Environment Influence on Phenotype
- Are identical twins always IDENTICAL? No
- nutrition, health care, money
In animals:
- Light - controls the color expression in some insects
- temperature - controls the sex of certain types of reptiles
- controls coloration in some mammals
- 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
X
Y
X
XX
XY
X
XX
Genotypic Ratio
Phenotypic Ratio
XY
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
- 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
- Example 1: Red eyes are dominant to white eyes in fruit flies. Cross a homozygous dominant redeyed female with a red-eyed male. Eye color is an X-linked trait.
X
X
X
Y
XX
XY
XX
XY
Genotypic Ratio
Phenotypic Ratio
- 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.
X
Y
X
XX
XY
X
XX
Genotypic Ratio
Phenotypic Ratio
XY
- carrier = does not show a disease/disorder but can pass it onto their offspring
- aka: heterozygous
Pedigrees
= a family record that shows how a trait is inherited over several generations
- boxes = males
circles = females
- shaded = person shows trait
not shaded = person doesn’t show the trait
half shaded = person is a carrier to the trait
- How to determine if the trait is on an autosome or a sex chromosome:
- If approximately the same number of males and females have the trait, it is most likely on the
autosome
- If it is more common in males, then the gene is likely on the X chromosomes
- If it occurs only in males, it is on the Y chromosome
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
2) monosomy = a zygote with 45 chromosomes
- has only one copy of a particular chromosome
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