1. Genetics Part II: Probability and Pedigree
Nestor T. Hilvano, M.D., M.P.H.
(Images Copyright Discover Biology, 5th ed., Singh-Cundy and Cain, Textbook, 2012.)

2. Learning Objectives
Explain the rule of multiplication and the rule of addition to determine the probability of an event in monohybrid crosses.
Explain how family pedigrees can help determine the inheritance of human traits.
Discuss and provide examples of how recessive and dominant disorders are inherited.
Describe the inheritance patterns of pleiotropy and polygenic inheritance.

3. Laws of Probability Govern Mendelian Inheritance
The probability of a specific event is the number of ways that event can occur out of the total possible outcomes.
# of actual occurrences
Probability = ______________________
# of possible occurrence
When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss
In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles
Ex: flipping heads on a coin= ½ (50%)

4. Rules of Probability Independent events – ex. Tossing of coin
Rule of Multiplication – probability of 2 events occuring together is the product of the probabilities of the 2 events occuring apart
Ex. Probability of recessive phenotype occuring in monohybrid? ½ x ½ = ¼
dihybrid? ¼ x ¼ = 1/16

5. Rule of Addition – probabilities are added if there is more than one way an outcome can occur, as in determining the chances for F2 heterozygous offspring
Ex. Probability of Bb is
¼ + ¼ = ½

6. Degrees of Dominance
Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical
Incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
Codominance, two dominant alleles affect the phenotype in separate, distinguishable ways; Type AB blood is an example

7. Incomplete dominance In snapdragon
Fig
P Generation
Red
White
CRCR
CWCW
Gametes CR CW
Incomplete dominance
In snapdragon
Pink
F1 Generation
CRCW
Gametes
1/2 CR
1/2 CW
Figure Incomplete dominance in snapdragon color
Sperm
1/2 CR
1/2 CW
F2 Generation
What is the Phenotype ratio? _____________
1/2 CR
CRCR
CRCW
Eggs
1/2 CW
CRCW
CWCW

8. Figure 12.11 Genetic Basis of the ABO Blood Types in Humans
The ABO blood types are determined by chains of sugars covalently attached to certain cell surface proteins on red blood cells. Type A red blood cells have a distinctive “A type” of sugar (chemically, N-acetylgalactosamine), whereas type B blood cells have a “B type” of sugar (galactose) at the end of the chain. The AB blood type reflects codominance: the red blood cells in type AB blood have about equal amounts of A-type sugars and B-type sugars on their cell surface. The I gene encodes an enzyme that comes in at least two allelic forms: the form encoded by the A allele adds A-type sugars to red blood cell surfaces, and the form encoded by the B allele adds B-type sugars. The i allele codes for a nonfunctional version of the enzyme that cannot attach any sugar to the cell surface protein. People who have two copies of the i allele (homozygotes) are said to have blood type O.

9. Pedigree Analysis
Figure 9.8A
Dominant Traits
Recessive Traits
Pedigree is a family tree that describes the interrelationships of parents and children
Inheritance patterns of particular traits can be traced and described
can be used to make predictions of future offspring
Dominant-recessive inheritance
Apply Mendelian laws
Freckles
No freckles
Widow’s peak
Straight hairline
Figure 9.8A Examples of single-gene inherited traits in humans
Free earlobe
Attached earlobe 9

10. Brachydactyly: dominant condition marked by short/clubbed fingers & toes
Figure 13.9 (Part 2) Patterns of Inheritance Can Be Analyzed in Family Pedigrees
The pedigree shown here illustrates symbols commonly used by geneticists. The Roman numerals at left identify different generations. Numbers listed below the symbols identify individuals of a given generation. This pedigree charts the inheritance of brachydactyly, a dominant condition marked by short or clubbed fingers and toes. Brachydactyly was the first genetic condition to be analyzed, in 1903, by following pedigrees.

11. Recessive Disorders
Most common; affected children are homozygous recessive
Born to normal parents who are both heterozygotes (Dd)
Ex. Deafness, albinism, cystic fibrosis, PKU, sickle-cell disease, etc.
Practice: construct punnett squares
Parents – both Dd (normal but are carriers)
Gametes: D and d
Probability for Hearing (DD, Dd)= ¾ (75%); Deaf (dd)= ¼ (25%); Carriers (Dd)= 2/4 (50%)

12. Dominant Disorders
Homozygous dominant causes death of embryo; only heterozygous have this disorder ; 50% chance of passing the condition
Ex. Achondroplasia (dwarf), extrafingers or web digits, Huntington’s disease, and Hypercholesterolemia

13. Achondroplasia: a dominant trait disorder
Fig
Achondroplasia: a dominant trait
disorder
Parents
Dwarf
Normal Dd  dd
Sperm D d
Eggs Dd dd
Figure Achondroplasia: a dominant trait d
Dwarf
Normal Dd dd d
Dwarf
Normal

14. Pleiotrophy and Polygenic Inheritance
Pleiotrophy = single gene produce multiple phenotypic characteristics.
ex: sickle cell disease (ss) – confers phenotypic traits as anemia, weakness, mental dysfunction, organ damage, etc.
Polygenic = 2 or more genes produce a single phenotypic characteristic.
ex: mixtures of 3 genes (each w/ 2 alleles) for phenotype of skin color

15. Crossing Over can separate linked alleles
Producing gametes with recombinant chromosomes (crossing over of linked genes) A B a b
Tetrad
Crossing over
Gametes

16. Sex-linked Disorders in Humans
Sex-linked genes determine the sex of individual and other inheritable characteristics
Sex-linked disorders are due to recessive alleles
For a recessive sex-linked trait to be expressed
A female needs two copies of the allele
A male needs only one copy of the allele
Sex-linked recessive disorders are much more common in males than in females
ex. Hemophilia (lack protein for blood clotting),
Color blindness (red-green), Duchenne
muscular dystrophy

17. Homework
Define terms: recessive disorders; dominant disorders; polygenic inheritance; pleiotrophy; pedigree; recombinant chromosomes, and sex-linked genes.
Describe the rule of multiplication and rule of addition in determining the probability of inheritance.
Male with black hair (Bb) married a female with red hair (bb). Construct a punnett square to show the location of gametes and offspring. What is the probability of having a black hair? Probability of red hair?
Whys is color blindness is more common in males?