1. Ch.5 Beyond Mendel’s Laws
5.1

2. Lethal Alleles Lethal genotypes- death before alleles are passed on.
Before reproduction.
Exception- Huntington’s disease-
Begins at age
Lethal alleles- spontaneous abortions
If both parents are heterozygous-
25 % chance of being homozygous recessive.
Homozygous dominant- lethal in embryos
Ex. Mexican hairless dogs.

3. Incomplete Dominance
Heterozygous phenotype is intermediate between each homozygote.
Human Ex.
TaySachs- intermediate enzyme level in heterozygotes.
Human hair curliness
SS=straight, ss=curly, Ss=wavy
FH-Familial hypercholesterolemia
A heterozygote has half the number or receptors for LDL cholesterol in the liver with intermediate plasma cholesterol levels.

4. Classic Snapdragons use C= color P1 CRCR x CWCW F1 CRCW x CRCW
RR= Red CRCR
rr= white(WW); CWCW
Rr (RW)= pink; CRCW
P1 CRCR x CWCW
F1 CRCW x CRCW
F2 CRCR CRCW CWCW 1:2:1
red pink white
Same genotypic and phenotypic
ratios

5. Codominance: Both alleles are experessed in a heterozygote.
AB Blood Type
ABO Blood types:
Determined by cell surface antigens
Antigen- protein with attached sugar on plasma membrane of red blood cells
Type A- A antigen
Type B- B antigen
Type O- no sugar antigen
Type AB- both A and B antigens

6. Codominance examples-
Both alleles are expressed equally
Example-
is red and white striped flowers
Roan cattle

7. Multiple alleles 1 gene; more than 2 allele forms Examples-
Acyl CoA dehydrogenase deficiency
Mitochondria- affects heart and skeletla muscles.
Other alleles lessen severity producing some enzyme.
PKU- buildup of phenylalanine
More than 300 mutant alleles-4 phenotypes
Classic PKU- profound mental retardation
Moderate PKU
Mild PKU
Excrete excess phenylalanine in urine
Correlation of genotypes with phenotypes – to monitor diet and predict outcome.

8. Cystic Fibrosis- Several hundred alleles 70% of cases- F508 homozygous
Many variations
Pancreatic function correlates better than lung symptoms.

9. Blood types Phenotypes- Blood Types (4) A B AB O Genotypes (6)
IAIA or IAi
IBIB or IBi
IAIB ii

10. Blood Typing: ABO blood group: Antigens are on the rbcs.
Antibodies to other antigen sugar types circulate in the plasma portion of blood.
Blood Type A- has anti B in plasma.
Blood Type B- has anti A in plasma
Blood Type O- has anti A,B in plasma
Blood type AB- no anti A or anti B
Blood Type O- universal donor because there are no A or B antigens.
Blood type AB- universal recipient, since no antibodies to A or B.
To type a person’s blood- the rbcs are mixed with different antibody sera. Clumping indicates the presence of the corresponding antigen on the red blood cells. Ex Antigen A clumps with antiA sera.

11. Rh Factor- Named for rhesus monkeys where found.
The main factor is a D antigen found on the red blood cells.
Unlike ABO blood types, antibodies are only present if an Rh negative person is exposed to Rh positive blood cells.
85 % pop. Has the D allele=Rh +
15% pop. Is recessive dd= Rh-
Rhogam- anti D is given to Rh- mothers to bind any fetal Rh + red blood cells if there is a bleed to prevent the mom from making circulating antibodies to the fetus’ red blood cells.
Hemolytic Disease of the newborn results from this imcompatibility. – can result in death.

12. Epistasis One gene affects the expression of a second gene
Example: H gene is epistatic to the ABO gene.
H protein attaches a molecule to the cell surface to which the A or B antigens are attached
hh genotype = no H protein=
Without H protein the A or B antigens can not be attached to the cell
All hh genotypes have the phenotype of type O, although the ABO blood group can be anything (A, B, AB, or O)

13. Bombay phenotype-identified in 1952- Bombay, India
Very rare example of epistasis
Individuals phenotype= Blood type O
However they have no H antigen so they have antibodies to H (anti-H), antibodies to A (anti-A) antibodies to B (anti-B) in their circulating plasma.
Bottom line- they will be incompatible with all blood types; even O
Can only receive their own blood; or another Bombay’s.

14. Incomplete Penetrance
The phenotype is not always observed among individuals carrying the genotype – express or not
DD or Dd - only 80% show polydactyly
Variable Expressivity
A phenotype that varies in intensity
 Polydactyly two extra digits on each hand and foot vs. one extra digit on one foot
 Individuals with the same genotype for familial hypercholesterolemia have varying levels of symptoms

15. Pleiotropy One gene has many symptoms or controls several functions
Example: porphyria variegata
Figure 5.5a
Figure 5.5b
Photo © North Wind Picture Archives

16. Genetic Heterogeneity
Different genes can produce identical phenotypes
 Hearing loss
 Osteogenesis imperfecta
Genes may encode for different enzymes in a biochemical pathway
 Clotting disorders
over 132 forms of autosomal recessive hearing loss

17. Phenocopy
Appears inherited but is caused by the environment
May have symptoms that resemble an inherited trait or occur within families
Examples:
Exposure to teratogens
Thalidomide causes limb defects similar to inherited phocomelia
Hydroquinone exposure looks like alkaptonuria
Infection
AIDS virus can be passed from mother to child, looking like it is inherited

18. Importance of Human Genome Sequence
Complications to Mendelian inheritance more common than originally thought
Overlapping of definitions – Marfan syndrome has both epistasis and genetic heterogeneity

19. Table 5.3

20. Mitochondrion Organelle providing cellular energy
Contains small circular DNA
No crossing over or DNA repair
High exposure to free radicals
Mutation rate is greater than nuclear DNA
37 genes without noncoding sequences
Mitochondrial genes are transmitted from mother to all of her offspring

21. Mitochondrial Inheritance
Figure 5.8
Mitochondrial DNA pedigree

23. Mitochondrial Disorder Examples
Mitochondrial myopathies – weak muscles
Leber optic atrophy – impairs vision
Ooplasmic transfer technique can enable woman to avoid transmitting a mitochondrial disorder

24. Heteroplasmy Many copies of the mitochondrial genome per cell
May have more than one allele for the same gene in the same cell
Heteroplasmy is the condition where mitochondrial DNA sequence is not the same in all copies