1. Extensions of the Laws of Inheritance
Chapter 8
Part B

2. Principles of Inheritance
Mendel studied traits that followed a simple pattern of dominant and recessive alleles for a single characteristic
Other modes of inheritance have also been described
These extensions of the laws of inheritance still follow the fundamental principles of Mendelian genetics
Genotypes are expressed differently for some of these crosses
The gene is named
For example: letter “A” could be used as the gene name
The gene name can still be used to indicate dominance depending on the type of inheritance
The alleles are added to the gene name as superscripts
ARAR, ARAr, ArAr

3. Incomplete Dominance
One allele isn’t fully dominant over the other allele, so the heterozygote’s phenotype is somewhere between the two homozygotes
Snapdragon flower color
Red flowers = RR
White flowers = rr
Heterozygotes = Rr
These are pink
The pigment produced by the red allele is diluted and thus appears pink
Mendel showed that traits don’t “blend”
This still holds true

4. Codominance Both alleles are simultaneously expressed in heterozygotes
Feather color gene A
Black allele AB
White allele AW
Genotype of the heterozygote: ABAW

6. Codominance
Difference between typical dominant/recessive, codominance, and incomplete dominance

7. Multiple Alleles There can be more than two alleles for a given gene
Blood type in humans has three alleles
IA codominant
IB codominant
i recessive

8. Multiple Alleles There can be more than two alleles for a given gene
Genotypes and Phenotypes
IAIA and IAi Type A blood
IBIB and IBi Type B blood
IAIB Type AB blood (codominantly expressed)
ii Type O blood

9. Epistasis
Some traits are the results of interactions of two or more gene pairs
At least eight genes contribute to eye color in humans
Not quite a simple as Mendel’s pea plant characteristics
Leads to continuous variation in traits

10. Epistasis
Some traits are the results of interactions of two or more gene pairs
Labrador coat color
Gene encoding pigment
black is dominant to brown
Gene encoding deposition of pigment
Dominant allele promotes deposition of pigment
Recessive allele reduces deposition
The two genes work together to determine how much of what color pigment ends up in the coat

12. Linked Genes
Mendel showed that genes on different chromosomes sort independently
Genes that are on the same chromosome are not inherited independently of each other
They are linked

13. Linked Genes
Linked genes can sort independently at a low frequency due to crossing over during meiosis

14. X-linked Traits The X and Y chromosomes carry different genes
Recessive alleles on the X chromosome create a unique pattern of inheritance

15. X-linked Traits More males affected
Each male receives one X and one Y chromosome
Two possible genotypes: XAY, XaY
Females receive two X chromosomes
Three possible genotypes: XAXA, XAXa , XaXa
Thus heterozygous males are affected while heterozygous females are not

16. X-linked Traits
Affected fathers cannot pass X-linked recessive alleles to a son
All children who inherit their father’s X chromosome are female

17. X-linked Traits
Affected fathers cannot pass X-linked recessive alleles to a son
Heterozygous females are the bridge between an affected male and his affected grandson

18. X-linked Traits Examples Red-Green color blindness Hemophilia A
Duchenne Muscular Dystrophy

20. X-linked Traits
X-linked dominant alleles that cause disorders are rarer because they tend to be lethal in male embryos

21. Human Inheritance Patterns
Inheritance patterns in humans are typically studied by tracking observable traits that crop up in families over many generations
Data is organized in pedigree charts
Chart symbols

23. Human Inheritance Patterns
Some easily observed human traits follow Mendelian inheritance patterns
Controlled by a single gene
Two alleles, one dominant and one recessive

24. Human Inheritance Patterns
Tongue rolling
Dominant condition (TT, Tt)
Ability to roll one’s tongue
Recessive condition (tt)
Inability to roll the tongue

25. Human Inheritance Patterns
Widow’s peak
Dominant condition (WW, Ww)
A distinct downward point in the frontal hairline
Recessive condition (ww)
A continuous hairline

26. Human Inheritance Patterns
Earlobe attachment:
Dominant condition (EE, Ee)
Detached or free earlobes
Recessive condition (ee)
Earlobes attached directly to the head

27. Human Inheritance Patterns
Autosomal Inheritance Patterns
Dominant Patterns
Dominant alleles are expressed in both homozygotes and heterozygotes
The trait specified tends to appear in every generation

28. Human Inheritance Patterns
Dominant Patterns
Examples
Achondroplasia
The allele interferes with formation of the embryonic skeleton
Adults average about 4’4” and have short limbs
Huntington’s disease
Nervous system slowly deteriorates
Involuntary muscle movements increase
Hutchison-Guilford Progeria
Drastic premature aging

29. Human Inheritance Patterns
Recessive Patterns
Expressed only in homozygous people
Traits may skip generations
Heterozygotes are carriers of the allele, but do not express the trait

30. Human Inheritance Patterns
Recessive Patterns
Examples
Albinism
Lack of melanin
Tay-Sachs disease
Malfunction of a lysosomal enzyme that breaks down gangliosides
Lipids accumulate to toxic levels in nerve cells
Cystic Fibrosis
Difficulty breathing
Chronic lung infections

31. Summary Extensions of the Laws of Inheritance Epistasis Linked Genes
Incomplete dominance
Codominance
Multiple Alleles
Epistasis
Linked Genes
X-linked traits
Human inheritance patterns