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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.)
Learning Objectives
1. Explain the rule of multiplication and the rule of addition to determine the probability of an event in monohybrid crosses.
2. Explain how family pedigrees can help determine the inheritance of human traits.
3. Discuss and provide examples of how recessive and dominant disorders are inherited.
4. Describe the inheritance patterns of pleiotropy and polygenic inheritance.
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%)
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
• 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
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
Fig. 14-10-3
Red
P Generation
Gametes
WhiteCRCR CWCW
CR CW
F1 GenerationPinkCRCW
CR CWGametes 1/21/2
F2 Generation
Sperm
Eggs
CR
CR
CW
CW
CRCR CRCW
CRCW CWCW
1/21/2
1/2
1/2
Incomplete dominance In snapdragon
What is the Phenotype ratio? _____________
Figure 9.8A
Dominant Traits Recessive Traits
Freckles No freckles
Widow’s peak Straight hairline
Free earlobe Attached earlobe
Pedigree Analysis
• 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
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%)
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
Fig. 14-17
Eggs
Parents
Dwarf Normal
Normal
Normal
Dwarf
Dwarf
Sperm
Dd dd
dD
Dd dd
ddDd
d
d
Achondroplasia: a dominant trait disorder
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
Crossing Over • can separate linked alleles
• Producing gametes with recombinant chromosomes (crossing over of linked genes)
A B
a b
Tetrad Crossing over
A B
A b
a b
a B
Gametes
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
Homework
1. Define terms: recessive disorders; dominant disorders; polygenic inheritance; pleiotrophy; pedigree; recombinant chromosomes, and sex-linked genes.
2. Describe the rule of multiplication and rule of addition in determining the probability of inheritance.
3. 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?
4. Whys is color blindness is more common in males?