1. Genes and Medical Genetics
Chapter 2

2. Genotype and Phenotype
Genotype = all the genes that an individual has
Phenotype = physical appearance of the individual
Pictures of Jennifer Aniston

3. Genotype (con’t)
Alleles can be dominant (capital letter) or recessive (lower case letter).
Alternate forms of a gene having the same position (locus) on a pair of matching chromosomes that control the same trait are called alleles
An individual has two alleles for each trait because a chromosome pair carries alleles for the same traits
How many alleles for each trait will be in the gametes?

4. Example – in humans unattached earlobe is dominant over attached earlobes
Could use capital E to show dominant allele and lower case e to show recessive (non-dominant) allele
Note: it is customary to designate alleles by the same letter, with uppercase to signify dominant and lowercase to signify recessive
Fig

5. When the two alleles are both dominant it is called a homozygous dominant genotype
The phenotype will be unattached earlobes
When the two alleles are both recessive it is called homozygous recessive
The phenotype will be attached earlobes
When the two alleles are different (one dominant, one recessive) it is called a heterozygous genotype
The phenotype will be ________________.
Fig

6. Forming gametes (gametogenesis)
Recall: gametes have ½ normal # of chromosomes
Happens when chromosome pair separates during meiosis
Since alleles are on chromosomes, they also separate during meiosis, thus gametes only carry one allele for each trait.
Ex. If individual was Ee, and produced four gametes, ½ would be E and ½ would be e.
(Chromosomes already doubled)
Fig Gametogenesis

7. Genotypes WW or Ww ww EE or Ee ee SS or Ss ss FF or Ff ff Phenotypes
Fig
Common
Inherited
Character
-istics in
Humans
Phenotypes

8. Genetic Crosses (One-trait Crosses)
If know genotype of parents, can predict chances of having a child with certain genotypes (and thus certain phenotypes).
Ex. If one parent is homozygous dominant (EE) the chance of having a child with unattached earlobes is 100% because parent only has dominant allele (E) to pass on to baby.
But, if both parents are homozygous recessive (ee), the chance of having a child with attached earlobes is 100% because parents only have recessive allele (e) to pass on to baby.

9. One-trait Crosses (con’t)
Fig Heterozygous-by- Heterozygous cross
If both parents are heterozygous (Dad is Ee, and Mom is Ee), what are the chances that baby will have unattached or attached earlobes?
Determine using a Punnett Square
Each child will have a 75% chance of dominant phenotype (unattached), and 25% chance of having recessive phenotype (attached).
Punnett Square

10. One-trait crosses (con’t)
Fig Heterozygous
By Homozygous cross
What if one parent is Ee and the other is ee?
Each child will have a 50% chance of having unattached earlobes
Punnett Square

11. Genetic Crosses (Two-trait Crosses)
What if want to look at two traits at the same time?
Ex. Widow’s peak (W) and short fingers (S) (both dominant traits).
If one parent is homozygous for widow’s peak and short fingers (WWSS) and other is homozygous for straight hairline and long fingers (wwss), what will
Children look like?
Grandchildren look like?

12. Fig. 02-06 Dihybrid cross using Punnett Square
Expected phenotypic ratio for a dyhibrid cross is always 9:3:3:1
Can use this expected ratio to predict chances of each child receiving a certain phenotype
Ex. Chance of getting two dominant phenotypes together is 9 out of 16
Chance of getting two recessive phenotypes is 1 out of 16

13. Autosomal Dominant Disorders
Genetic disorders are caused by mutations
Mutations – permanent changes in genes (DNA)
If disorder is autosomal dominant  mutation is a single allele, and heterozygotes will exhibit the disorder
If know genotype of parents, can determine chances of children having the disorder (see Table 2.2, p. 26)
Can provide genetic counseling to parents who can make decision about best courses of action (Chapter 4).

14. Autosomal Dominant Disorders
Neurofibromatosis
Most common genetic disorders (1 in 3,500 newborns, all races and ethnicities)
Symptoms
Large tan spots on skin that get darker with age
Small, benign (?) tumors (neurofibromas) often occur in nerves
If severe case
Skeletal deformities (including a large head)
May develop eye and ear tumors  become blind and deaf
Children with non-severe symptoms may still have learning disabilities and be hyperactive
Gene for neurofibromatosis has been located on chromosome 17
Found that gene controls production of protein that usually blocks cell growth
If gene not working  certain cells grow out of control =_______________ .

15. Autosomal Dominant Disorders
Huntington Disease
Neurological (nervous system) disorder that causes progressive degeneration of brain cells.
Symptoms
Most patients appear normal until middle age (may already have children)
Severe muscle spasms
Personality disorders
Treatment – none (patients die in 10 – 15 years after symptoms appear)
Gene (mutated) located on chromosome 4  causes abnormal protein that clumps inside neurons (nerve cells)
Test developed for presence but most people do not want to know
Two minute paper: “Would you want to be tested for a fatal genetic disease?
Why or why not?”

16. Autosomal Recessive Disorders
Cystic Fibrosis (CF)
Most common lethal genetic disease among Caucasians in the US
1 in 20 are a carrier, 1 in 2,500 newborns has it
Caused by defect in plasma membrane (gene on chromosome 7)
Symptoms
Thickened mucous in bronchial tubes (problems breathing) and pancreatic duct (problems digesting)
Treatment – mucous in lungs manually loosened and other treatments have raised life expectancy to years old

17. Fig Treatment for CF

18. Autosomal Recessive Disorders
Phenylketonuria (PKU)
Not so common as CF
Affects nervous system development
Caused by missing enzyme that normally allows metabolism of the amino acid phenylalanine
Causes abnormal breakdown product (phenylketone) in urine
Symptoms – severe mental retardation, black urine
PKU allele located on chromosome 12  prenatal DNA test can detect it (and elevated phenylalanine in blood)
If detect  place newborn on diet low in phenylalanine until at least age 7

19. Autosomal Recessive Disorders
Tay-Sachs Disease
Usually occurs in Jewish people
Symptoms
Development slows at age 4 to 8 months
Neurological and Psychomotor impairment
Child gradually becomes blind and helpless, seizures, paralyzed, death by age 3 – 4 years old
Caused by gene on chromosome 15  caused buildup of nonfunctional lysosomes in neurons

20. Autosomal Recessive Disorders
Albinism
Person unable to produce pigment melanin  colors skin, hair, eyes
Symptoms
White hair, pink eyes, super-white skin
Vision problems
Example of epistasis  one gene affects the expression of other genes
In albino  any gene for coloring cannot be expressed because mutated gene prevents them from producing melanin
Picture of Johnny Winter