1. Chapter 20 Patterns of Genetic Inheritance
Mader, Sylvia S. Human Biology. 13th Edition. McGraw-Hill, 2014.

2. Points to Ponder
What is the genotype and the phenotype of an individual?
What are the genotypes for a homozygous recessive and dominant individuals and a heterozygote individual?
Be able to draw a punnett square for any cross (1-trait cross, 2-trait cross and a sex-linked cross).
What are Tay-Sachs disease, Huntington disease, sickle-cell disease, and PKU?
How are each of the above inherited?
What is polygenic inheritance?
What is a multifactorial trait?
What is sex-linked inheritance?
Name 3 X-linked recessive disorders.
What is codominance?
What is incomplete dominance?
What do you think about genetic profiling?

3. These traits are genetically inherited
20.1 Genotype and phenotype
These traits are genetically inherited
Answer these questions about your inheritance:
Do you have a widow’s peak?
Are your earlobes attached or unattached?
Do you have short or long fingers?
Do you have freckles?
Can you roll your tongue?
Do you have Hitchhiker’s thumb?

4. Genotype
Genotype – specific genes for a particular trait written with symbols
Alleles:
alternate forms of a specific gene at the same position (locus) on a gene (e.g. allele for unattached earlobes and attached lobes)
alleles occur in pairs
Dominant gene: will be expressed and will mask a recessive gene (Tt or TT)
Recessive allele: allele that is only expressed when a gene has two of this type of allele

5. Genotype Genotype Homozygous dominant genotype:
2 dominant alleles (TT or AA)
Homozygous recessive genotype:
2 recessive alleles (tt or aa)
Heterozygous genotype:
one dominant allele and one recessive allele (Tt or Aa)

6. 20.1 Genotype and phenotype
Phenotype – the physical or outward expression of the genotype
Genotype Phenotype
EE unattached earlobe
Ee unattached earlobe
ee attached earlobe
What are your genotype and phenotype?

7. Understanding genotype & phenotype

8. What about your inheritance?

9. Crosses
One-trait cross – considers the inheritance of one characteristic
e.g. WW x Ww
Two-trait cross – considers the inheritance of two characteristics
e.g. WWTT x WwTT
Gametes only carry one allele, so if an individual has the genotype Ww what are the possible gametes that this individual can pass on?
Answer: either a W or a w but not both
Another example:

10. Punnett squares Punnett squares
20.2 One-and Two-trait inheritance
Punnett squares
Punnett squares
use of a grid that diagram crosses between individuals by using the possible parental gametes
These allow one to figure the probability that an offspring will have a particular genotype and phenotype

11. Practicing punnett squares
20.2 One-and Two-trait inheritance
Practicing punnett squares
eggs
What would a punnett square involving a man (M) with a genotype Ff and a women (F) with a genotype Ff look like?
F – freckles
f – no freckles
M/F F f FF Ff ff
sperm

12. Practicing ratios Genotypic ratio: Phenotypic ratio:
number of offspring with the same genotype
Phenotypic ratio:
number of offspring with the same outward appearance
What is the genotypic ratio?
1: 2: 1 (1 FF: 2 Ff: 1 ff)
What is the phenotypic ratio?
3: 1 (3 with freckles and 1 with no freckles)
eggs
M/F F f FF Ff ff
sperm

13. 20.2 One-and Two-trait inheritance
Monohybrid crosses
Monohybrid cross – an experimental cross in which parents
are identically heterozygous at one gene pair (e.g. Aa x Aa)
One Trait Crosses

14. Possible gametes for two traits All Possible Combinations of Chromosomes and alleles in the gametes (via meiosis)

15. Dihybrid cross (a type of two-trait cross)
experimental cross usually involving parents that are homozygous for different alleles of two genes and results in a 9:3:3:1 genotypic ratio for the offspring

16. Practicing a punnett square for 2-trait cross
What would the punnett square look like for a dihybrid cross between a male that is WWSS and a female that is wwss?

17. Phenotypic Ratios of Common Crosses
Genotype
Phenotype
Monohybrid x Monohybrid
(Ww x Ww)
3:1 (dominant to recessive)
Monohybrid x recessive
(Ww x ww)
1:1 (dominant to recessive)
Dihybrid x Dihybrid
(WwSs x WwSs)
9:3:3:1 (9 both dominant, 3 one dominant, 3 other dominant, 1 both recessive
Dihybrid x recessive
1:1:1:1 (all possible combinations)

18. Family Pedigrees for Genetic Disorders
Autosomal Dominant
Individual with alleles AA or Aa will have disorder
Autosomal Recessive
Only individuals with alleles aa will have disorder
Key:
Square = male
Circle = female
Shaded circle/square = affected individual
Line between square and circle = union
Vertical line going downward = child/children

19. Autosomal recessive disorder
Individuals must be homozygous recessive to have the disorder

20. Autosomal dominant disorder
Individuals that are homozygous dominant and heterozygous will have the disorder

21. Autosomal Recessive Disorders
Tay-Sachs
Cystic Fibrosis
Phenylketonuria
Sickle-cell Disease
Huntington Disease

22. Genetic disorders of interest
Tay-Sachs disease:
lack of the enzyme that breaks down lipids in lysosomes
results in membranous cytoplasmic bodies (MCB) in the cells present in the cortical neuron
eventually death of a baby
Cystic fibrosis:
Cl- do not pass normally through a cell membrane, so Na+ and water can not enter the cell
results in thick mucus in lungs and causes infections, clogs pancreatic ducts preventing function of digestive enzymes
Phenylketonuria (PKU):
lack of an enzyme needed to make a certain amino acid
affects nervous system development

23. Genetic disorders of interest
Sickle-Cell disease:
red-blood cells are sickle shaped rather than biconcave that clog blood vessels
Sickle-cell heterozygotes have sickle-cell traits in which the blood cells are normal unless they experience dehydration or mild oxygen deprivation
Huntington disease:
Caused by a mutated copy of the gene (on chromosome 4) for a huntingtin protein resulting in too many glutamine amino acids
leads to progressive degeneration of brain cells

24. Genetic disorders Huntington Disease Tay sachs Cystic fibrosis
20.2 One-and Two-trait inheritance
Genetic disorders
Huntington Disease
Tay sachs
Cystic fibrosis

25. Polygenic inheritance
20.3 Beyond simple inheritance
Polygenic inheritance
Polygenic traits - two or more sets of alleles govern one trait
Each dominant allele codes for a product so these effects are additive
Results in a continuous variation of phenotypes
Environmental effects cause intervening phenotypes
e.g. skin color ranges from very dark to very light
e.g. height vary among
Multifactorial trait – a polygenic trait that is particularly influenced by the environment
e.g. skin color is influenced by sun exposure
e.g. height can be affected by nutrition

26. Polygenic inheritance Distribution of phenotypes expected to follow a bell-shaped curve

27. Demonstrating environmental influences on phenotype
20.3 Beyond simple inheritance
Demonstrating environmental influences on phenotype
Himalayan rabbit’s coat color influenced by temperature
There is an allele responsible for melanin production that appears to be active only at lower temperatures
The extremities have a lower temperature and thus the ears, nose paws and tail are dark in color

28. 20.3 Beyond simple inheritance
Incomplete dominance
Occurs when the heterozygote is intermediate between the 2 homozygotes
Example:
(curly hair) CC x SS (straight hair)
CS (wavy hair)

29. 20.3 Beyond simple inheritance
Codominance
Occurs when the alleles are equally expressed in a heterozygote
Example:
(Type A blood) AA x BB (Type B blood)
AB (Type AB blood that has characteristics
of both blood types)

30. Multiple allele inheritance
20.3 Beyond simple inheritance
Multiple allele inheritance
The gene exists in several allelic forms
A person only has 2 of the possible alleles
A good example is the ABO blood system
A and B are codominant alleles
The O alleles is recessive to both A and B therefore to have this blood type you must have 2 recessive alleles

31. Multiple allele inheritance
20.3 Beyond simple inheritance
Multiple allele inheritance
Based on what you know what type of blood would each of the following individuals have in a cross between Ao and Bo?
possible genotypes: phenotypes:
AB Type AB blood
Bo Type B blood
Ao Type A blood
oo Type O blood

32. Blood type inheritance
20.3 Beyond simple inheritance
Blood type inheritance

33. Sex-linked inheritance
Traits are controlled by genes on the sex chromosomes
X-linked inheritance:
allele is carried on the X chromosome
Y-linked inheritance:
allele is carried on the Y chromosome
Most sex-linked traits are X-linked

34. X-linked inheritance: Color blindness Carried on the X chromosome
20.4 Sex-linked inheritance
X-linked inheritance: Color blindness Carried on the X chromosome
Cross:
XBXb x XBY
Possible offspring:
XBXB normal vision female
XBXb normal vision female
XBY normal vision male
XbY normal vision male

35. X-linked disorders
More often found in males than females because recessive alleles are always expressed
Most X-linked disorders are recessive:
Color blindness:
most often characterized by red-green color blindness
Muscular dystrophy:
characterized by wasting of muscles and death by age 20
Hemophilia:
characterized by the absence of particular clotting factors that causes blood to clot very slowly or not at all

36. X-linked disorders

37. X-linked disorders: Hemophilia

38. Bioethical focus: Genetic profiling
20.4 Sex-linked inheritance
Bioethical focus: Genetic profiling
Genetic profiling is a way to look for genetic disorders that you may have now or in the future
Discrimination concerns:
Could insurance companies use this to increase rates or not insure you?
Could an employer not hire you based on this knowledge?
Employer concerns:
Could the government mandate they provide an environment for each employee’s need in order to prevent illness or should employees be required to move from an area or job that could cause future disease?
Public benefits:
Most believe this could lead to better preventative care
Having this type of information can allow complex studies that can further our understanding of disease