1. Chapter 11: Complex Inheritance and Human Heredity

2. 11. 1 Basic Patterns of Human Inheritance 11
11.1 Basic Patterns of Human Inheritance 11.2 Complex Patterns of Inheritance

3. EQ: What is the difference between sex linked genes and genes found on autosomal chromosomes?

4. Chromosomes and Phenotype
KEY CONCEPT: The chromosomes on which genes are located can affect the expression of traits.
Gene expression is often related to whether a gene is located on an autosome or on a sex chromosome.
All humans have 46 chromosomes
23 pairs
44 are autosomes (“normal” traits)
Pairs 1-22
2 are sex chromosomes (X or Y)
XX = female XY = male
Sex chromosomes determine an organism’s sex
Autosomes are the other chromosomes that DO NOT play a direct role in sex determination.

5. Mendel Mendel studied autosomal gene traits, like hair texture.
Mendel’s rules of inheritance apply to autosomal genetic disorders.
A heterozygote for a recessive disorder is a carrier.
Disorders caused by dominant alleles are uncommon.
(dominant)
protein cystic fibrosis transmembrane conductance regulator (CFTR). This protein is required to regulate the components of sweat, digestive fluids, and mucus.
Leads to thick, viscous secretions

6. Sex linked genes Males and females can differ in sex-linked traits.
Genes on sex chromosomes are called sex-linked genes.
Y chromosome genes in mammals are responsible for male characteristics.
X chromosome genes
in mammals affect many traits.
X chromosomes carry some genes that Y does not.
Y is not large enough to carry them
Cause a difference of inheritance between males and females

7. Sex linked genes cont. Male mammals have an XY genotype.
All of a male’s sex- linked genes are expressed.
Males have no second copies of sex- linked genes.

8. 7.1 Sex linked genes cont. Female mammals have an XX genotype.
Expression of sex-linked genes is similar to autosomal genes in females.
X chromosome inactivation randomly “turns off” one X chromosome.
XO =Orange Fur
Xo = Black fur

9. An in-depth look
The inactive X chromosome is silenced by it being packaged in such a way that it has a transcriptionally inactive structure called heterochromatin (silencing of genes)
X-inactivation prevents them from having twice as many X chromosome gene products (RNA or proteins) as males, which only possess a single copy of the X chromosome.
Abnormal amounts of gene products can be related to oncogenes which cause cancer.
The choice of which X chromosome will be inactivated is random in placental mammals such as humans, but once an X chromosome is inactivated it will remain inactive throughout the lifetime of the cell and its descendants in the organism.

10. Sex linked genes cont.
Most of the sex linked genes are found only on the X chromosome
Very few are found on the Y chromosome
Cause difference in male & female inheritance
Males will be affected or unaffected only
Females will be affected, unaffected, or a carrier for these traits.

11. Examples of X-linked traits
Found on the X chromosomes: red-green color blindness, muscular dystrophy, baldness (male pattern baldness), hemophilia
Found on the Y: SRY gene (maleness gene)
Many genes that code for proteins are found on the Y gene

12. Notice male chance is 50% and female chance is 50%
Female carriers can pass on a trait without showing that trait themselves

13. Human Genetics and Pedigrees
KEY CONCEPT: A combination of methods is used to study human genetics.
Human genetics follows the patterns seen in other organisms.
The basic principles of genetics are the same in all sexually reproducing organisms.
Inheritance of many human traits is complex.
Single-gene traits are important in understanding human genetics.

14. Gene Expression Females can carry sex-linked genetic disorders.
Males (XY) express all of their sex linked genes.
Expression of the disorder depends on which parent carries the allele and the sex of the child. X Y

15. EQ: What is a Pedigree Chart and what is its significance?

16. Pedigree Charts A pedigree is a chart for tracing genes in a family.
What you need to know in order to read a pedigree chart.

17. Pedigree Charts Phenotypes are used to infer genotypes on a pedigree.
Autosomal genes show different patterns on a pedigree than sex-linked genes.

18. Pedigree Charts cont...
If the phenotype is more common in males, the gene is likely sex-linked.

19. EQ: What role does complex patterns of inheritance play in organisms?

20. 11.2 Complex Patterns of Inheritance
KEY CONCEPT: Phenotype is affected by many different factors.
The rules are not always followed!
There are some exceptions
Phenotype can depend on interactions of alleles.

21. In incomplete dominance- heterozygous genes have a phenotype that is between the dominant and recessive traits
Black + White = gray
Also called intermediate inheritance
Heterozygous phenotype is intermediate between the two homozygous phenotypes
Homozygous parental phenotypes not seen in F1 offspring

22. Incomplete dominance

23. Incomplete dominance in humans
Face shape
Ear size
Type of hair (curly, wavy. Straight)
Nose size
Mouth size

24. Codominance
Codominant alleles will both be completely expressed (both dominant).
Codominant alleles are neither dominant nor recessive.
The ABO blood types result from codominant alleles.

25. Multiple Alleles Multiple Alleles
More than 2 alleles may exist for a trait
Blood Type – A, B, O, & AB types
IA, IB, i (3 alleles)
Type AB
DO NOT mask over each other

26. Variations Many genes may interact to produce one trait.
Polygenic Inheritance – when two or more genes affect a trait
Examples: height, skin color, eye color
Multiple genes contribute to the trait
AaBbCc – trait using three sets of genes
Some traits can be affected by dozens of genes

27. Polygenic Inheritance
Order of dominance: brown > green > blue.

28. In humans, eye color is the result of polygenic inheritance
In humans, eye color is the result of polygenic inheritance. At the present, three gene pairs controlling human eye color are known. Two of the gene pairs occur on chromosome pair 15 and one occurs on chromosome pair 19. The bey 2 gene, on chromosome 15, has a brown and a blue allele. A second gene, located on chromosome 19 (the gey gene) has a blue and a green allele. A third gene, bey 1, located on chromosome 15, is a central brown eye color gene.

29. The brown allele is always dominant over the blue allele so even if a person is heterozygous (one brown and one blue allele) for the bey 2 gene on chromosome 15 the brown allele will be expressed. The gey gene also has two alleles, one green and one blue. The green allele is dominant to the blue allele on either chromosome but is recessive to the brown allele on chromosome 15. This means that there is a dominance order among the two gene pairs. If a person has a brown allele on chromosome 15 and all other alleles are blue or green the person will have brown eyes. If there is a green allele on chromosome 19 and the rest of the alleles are blue, eye color will be green. Blue eyes will occur only if all four alleles are for blue eyes.

30. Heterochromia iridum
In complete heterochromia, one iris is a different color from the other. In partial heterochromia or sectoral heterochromia, part of one iris is a different color from its remainder.
Most cases of heterochromia are hereditary, caused by a disease or syndrome, or due to an injury.
The pigment is produced in a specialized group of cells known as melanocytes.

31. Alexandria’s Genesis Not Enough Scientific Evidence to support
It is possible that there is a gene variation that can cause purple eyes, lighter skin and allows people to remain healthy so they live longer, but there is not necessarily enough evidence that this specific condition exists today.
The traits of Alexandria’s Genesis appear to be too widespread to be part of a single gene, which means they would have to stem from several unrelated genes. Some of the traits associated with this disorder are physiologically impossible.
One could not fail to produce waste because toxins would build up in the body, causing death.
Fair skin that resists burning is also impossible because the only way to protect the skin is by the presence of melanin that makes the skin darker.

32. Melanocytes/ Melanin
Primarily responsible for skin color, also for eye color, and hair color.
The difference in skin color between lightly and darkly pigmented individuals is due not to the number (quantity) of melanocytes in their skin, but to the melanocytes' level of activity
Tyrosinase is required for melanocytes to produce melanin from the amino acid tyrosine.

33. Additional information… Eumelanin and Pheomelanin
Eumelanin: There are two types are recognized: black and brown. Black melanin is darker than brown. A small amount of black eumelanin in the absence of other pigments causes grey hair. A small amount of brown eumelanin in the absence of other pigments causes yellow (blond) color hair.
Pheomelanin: imparts a pink to red hue and, thus, is found in particularly large quantities in red hair. Pheomelanin is particularly concentrated in the lips and nipples.
How they differ from each other is their chemical structure.

34. Albinism
Albinism results from inheritance of recessive gene alleles and is known to affect all vertebrates, including humans.
Albinos lack an enzyme called tyrosinase.

35. Environmental Factors
The environment interacts with genotype.
Phenotype is a combination of genotype and environment.
The sex of sea turtles depends on both genes and the environment (temperature dependent- sex determination)
Occurs in embryonic stage. Males are produced at lower temperatures than females.
Height is an example of a phenotype strongly affected by the environment.