HUMAN GENETICS Lab Below is a listing which will help you to prepare for the quiz and lab on this material.


I.Review from Previous Lab:

• GENOTYPE = the genetic make-up of the individual • PHENOTYPE = the physical manifestation of the genotype (what the organism shows)

--note: letters are used to represent gene pairs

o Dominant allele - A gene which can hide or mask the second gene in a pair

o Recessive allele - gene which can be masked

Ex: W - dominant - widow's peak w - recessive - straight hairline

3 possible Genotypes:

WW - homozygous dominant

Ww - heterozygous

ww - homozygous recessive

Monohybrid Crosses - Genotypes of the parents:

When you cross a homozygous dominant (WW) with a homozygous recessive (ww), the results are always:

genotypic ratio - 100% Ww phenotypic ratio - 100% dominant phenotype

(widow's peak)

Cross two parents that are heterozygous for hairline:

II. Chromosome Variations In Humans

Autosomal Recessive Inheritance –

1. Defective Gene is carried recessively 2. Males or females carry the disorder on a non-sex chromosome (autosome) 3. Ex: Galactosemia 4. Make a genotype/phenotype table for all problems:

3 possible genotypes for Galactosemia: Genotype Phenotype AA Normal Aa* Carrier a*a* Galactosemic Note: (*) represents the defective allele Problem: Cross a normal homozygous dominant parent with a carrier for Galactosemia Cross AA x Aa*

A A A AA AA a* Aa* Aa* Results: 50% genetically normal; 50% carriers; 100% APPEAR normal

Autosomal Dominant Inheritance

1. Males or females carry the defect on a non-sex chromosome (on an autosome) 2. Trait is dominant 3. Example: Huntington's Disorder - a progressive degeneration of the nervous system

(onset > 40 yrs) 4. Example: Achondroplasia (dwarfism) 5. Make a table for Autosomal Dominant Disorders

Genotype Phenotype A*A* Huntington’s A*a Huntington’s aa Normal

Note: (*) indicates the defective gene

Problem : Cross a normal parent with a homozygous dominant individual with Huntington's.

aa x A*A*

a a A* A*a A*a A* A*a A*a How many of the offspring will die from Huntington's Disorder? 100%

III. Sex Chromosomes - determine the sex of the fetus XX - 2 X chromosomes = FEMALE Xy - one X, 1 y chromosome = MALE

Sex Determination Mother:

XX Possible gametes from mom: X, X

Father: Xy Possible gametes from dad: X, y

Sex determination 50 % male 50% female X X


y Xy Xy

X-linked recessive Inheritance –

1. The mutated gene is on the X chromosome (not found on the y) 2. Carried recessively, so a normal X can mask it 3. Ex. Hemophilia - caused by a gene for clotting factor VIII which is mutated (causes

uncontrolled bleeding)

Genotype/Phenotype Table: (Must show both males and females)

Xy Normal X*y Hemophiliac Females XX Normal XX* Carrier X*X* Hemophiliac

Problem: Cross a carrier female with a normal male.

XX* x Xy X X* X XX XX* y Xy X*y

Results: 25% of ALL offspring are hemophilicas 50% of FEMALES are carriers 50% of MALES are hemophiliacs