Compendium Review Chapter 20

Table of Contents

I. Genotype and Phenotype
II. One and Two Trait Inheritance
III. Family Pedigrees for Genetic Disorders
IV. Genetic Disorders of Interest
V. Beyond Simple Inheritance Patterns
VI. Sex-Linked Inheritance


I. Genotype and Phenotype

A. Genotype: the genes of an individual.
1a. Allele: alternative form of a gene. "A particular gene, or protein-coding region of DNA along a chromosome might have a few different variations, called alleles. The combination of alleles, at a particular gene, or chromosome region, that you get from your mother and father determine your hereditary traits" (Frolich PowerPoint slide 46)

- Dominant: Uppercase letter. Recessive: Same letter, but lowercase. Occur in pairs, so a person normally has two alleles per trait. Ex. Homozygous dominant genotype = EE. Ex2. Homozygous recessive genotype = ee. Ex3. Heterozygous genotype = Ee. Dominant takes recessive. Must have two recessive to have recessive phenotype.

B. Phenotype: The physical appearance of the individual. (Mader 422)

II. One and Two Trait Inheritance

A. Genetic traits combine and pass on from parent to offspring in the following manner:
1a. During meiosis, the homologous chromosomes separate, and their number of chromosomes is reduced.

(Insert meiosis overview picture / Frolich PowerPoint slide 45)

2a. Since alleles are located on the chromosome, they separate too during meiosis, so the gamete carries only one allele for each trait. If the alleles were both dominant, the gametes would carry a dominant also. If they were both recessive, the gametes would carry a recessive. If there was one of each (a dominant and a recessive), Half the gametes would carry a dominant, and half would carry a recessive. (Mader 423)

3a. Fertilization results in union of female gamete (egg) with male gamete (sperm).

4a. Subsequent embryonic, fetal and embryonic development by mitosis and differentiation of cell types produces new individual. (Frolich PowerPoint Slide 44)

B. One-Trait Crosses: only one set of alleles is being considered.
1b. Monohybrids: heterozygous with regard to one pair of alleles.
2b. Monohybrid cross: phenotype ratio is always expected to be 3:1

(Insert monohybrid cross picture / http://www.bio200.buffalo.edu/ http://rds.yahoo.com/_ylt=A0S020rqllRIlfIAn_CjzbkF/SIG=12884tujr/EXP=1213589610/**http%3A//www.bio200.buffalo.edu/labs/heritability.html )

C. Punnett square and Probability
1c. Punnett square allows you to figure the probability that an offspring will have a particular genotype/phenotype.
- Two laws or probability: Product rule and the sum rule
-Product rule: the chance of two different events occurring together is the product (multiplication) of their chance occurring separately.
- Sum rule: The chance of any event that can occur in more than one way is the sum (addition) of the individual chances.

D. Two-Trait Crosses: Two sets of alleles are considered.
1d. There are no restrictions as to which homologue goes into which gamete. In the end, the gametes will collectively have all possible combinations of alleles.
2d. The process of meiosis explains why a person with the genotype EeFf would produce the gametes EF, ef, Ef, and eF in equal number.
3d: Probability: Calculate the probable outcome of dihybrid cross by multiplying the individual fractions.

E. Dihybrid Cross: Genotype that occurs when the individual is heterozygous in two regards. Ratio is always 9:3:3:1, when simple dominance is present. (Mader 426-427)

III. Family Pedigrees for Genetic Disorders

A. Genetic counselors often make pedigrees to see whether a condition that runs in the family is dominant or recessive. A pedigree shows the pattern of inheritance for a particular condition.

B. A person is considered a carrier if he or she appear to be normal but are capable of having a child with a genetic disorder. (Ex. Condition is recessive, and parents are Aa and Aa. Child gets aa = genetic disorder.)

C. Autosomal recessive disorders:
1c. Affected children can have unaffected parents.
2c. Heterozygotes (Aa) have an unaffected phenotype.
3c. Two affected parents will always have affected children.
4c. Affected individuals with homozygous unaffected mates will have unaffected children.
5c. Close relatives who reproduce are more likely to have affected children.
6c. Both males and females are affected with equal frequency.

D. Autosomal dominant disorders:

1d. Condition is dominant. Parents can be Aa (heterozygous). If child inherits one recessive allele from each parent, he /she is unaffected.
2d. Affected children will have an affected parent.
3d. Heterozygoes Aa are affected.
4d. Two unaffected parents will not have affected children.
5d. Both males and females are affected with equal frequency.

(Mader 429)


IV. Genetic Disorders of Interest

A. Autosomal Recessive Disorders: require two recessive alleles.

(Insert autosomal recessive disorder chart / http://www.justin.aekr.nl/ / http://rds.yahoo.com/_ylt=A0S0207KpVRIIQ4AZ0OjzbkF/SIG=11u1cvsg1/EXP=1213593418/**http%3A//www.justin.aekr.nl/Tay-Sachs_1.html )
1a. Tay-Sachs Disease: Most common among Jewish people, most of whom are of central and eastern European descent. Results from a lack of enzyme hexosaminidase A and the storage of its substrate, a glycosphingolipid in lysosomes. Primary sites of storage of lysosomes are in the brain, which equals progressive deterioration. Development slows between 4 and 8 months, when neurological impairment and psychomotor difficulties are then noticed. Child eventually goes blind, has seizures, and is eventually paralyzed.

(Insert brain w/ Tay-Sachs picture / erl.pathology.iupui.edu http://erl.pathology.iupui.edu/C603/LABE103.HTM)

2a. Cystic Fibrosis: Most common among Caucasians in US. Chloride ions fail to pass through plasma membrane channel protein in the cells. Creates lack of water. Creates mucus in bronchial tubes and pancreatic ducts that interferes with the function of the lungs and pancreas. Frequent lung infections.

(Insert cystic fibrosis picture / http://www.wdsu.com/
ttp://www.wdsu.com/encyclopedia/6867098/detail.html)

3a. Phenylketonuria: Metabolic disorder that affects nervous system development. Lack enzyme needed for normal metabolism of amino acidphenylalanine, and therefore, it appears in the urine and blood. Special diet helps for a normal development, but if not adhered to, can result in severe retardation.

(Insert phenylketonuria picture /www.newbornscreening.info http://rds.yahoo.com/_ylt=A0S0204IqlRIISEARC.jzbkF/SIG=12b3vp0lp/EXP=1213594504/**http%3A//www.newbornscreening.info/tools/GraphicsLib.html )

4a. Sickle-Cell Disease: Red blood cells are not bi-concave disks like normal red blood cells. They are irregular. Caused by an abnormal hemoglobin that differs from normal hemoglobin by one amino acid in the protein globin. Can clog vessels and break down. Causes poor circulation, anemia, and low resistance to infection.
(Mader 430-431)

(Insert sickle cell disease picture /www.learningsupport.co.uk/ http://www.learningsupport.co.uk/resources6.html)


B. Autosomal Dominant Disorders: Inheritance of only one dominant allele is necessary for an autosomal dominant genetic disorder.
1b. Marfan Syndrome: Caused by a defect in an elastic connective tissue protein, called fillibrin. This protein is normally abundant in the lens of the eye, the bones of the limbs, fingers and ribs, and in the wall of the aorta. Aorta wall is weak and can burst without warning. Tissue graft may help.

(Insert marfan picture / http://www.surgery.wisc.edu/ http://www.surgery.wisc.edu/cardio/adultcongenital/marfan.shtml)

2b. Huntington Disease: neurological disorder that leads to progressive degeneration of brain cells. Caused by a mutated copy of the gene for protein, called huntingtin. No effective treatment, death comes approx. 10-15 years after the onset of symptoms.
(Mader 431)

V. Beyond Simple Inheritance Patterns

A. Polygenic traits: Traits like skin color and height, governed by several sets of alleles. Results in continuous variation of phenotypes.
1a. Skin color: multifactorial trait: a polygenic trait that is particularly influenced by sun exposure.
2a. Multifactorial disorders: Polygenes that are subject to environmental influences are likely the cause of many human disorders, like cleft lip and clubfoot.

B. Incomplete Dominance and Codominance
1b. Incomplete dominance occurs when the heterozygote is intermediate between the two homozygotes. Ex. When a curly-haired person reproduces with a straight-haired person and the child has wavy hair. Only one allele codes for a product and the single dose of the product gives the intermediate result.
2b. Codominance: Occurs when alleles are equally expressed in a heterozygote. Ex. Blood type AB, in which red blood cells have the characteristics of both type A and type B blood.
(Mader 434)

C. Multiple Allele Inheritance
1c. When a trait is controlled by multiple alleles, the gene exists in several allelic forms.
(Mader 435)

VI. Sex-Linked Inheritance
A. One pair of the 23 pairs of chromosomes are the sex chromosomes (differ between the sexes). The other 22 pairs are autosomes.
B. Traits controlled by genes on the sex chromosomes are sex-linked: an allele on an X chromosome is x-linked, one on the Y is Y-linked.
C. A male always receives an x-linked allele from his mother, from whom he inherited an X chromosome. The Y chromosome from the father does not carry an allele for the trait.
D. Usually, a sex-linked genetic disorder is recessive.
E. X-linked disorder: Most sex-linked disorders are usually carried on x chromosome.
1e. X-linked disorders: color blindness, muscular dystrophy (wasting away of the muscles), hemophilia ("bleeder's disease"). (Mader 437)
F. Some combinations of alleles, produced during meiosis and fertilization might be more advantageous
This is what leads to natural selection. Individuals with more advantageous traits will survive to reproduce and pass on those traits.
Darwin realized that slow changes in inherited traits, due to natural selection produced the great evolutinoary history of life.
Before his synthesis of all the evidence, no one could make sense of living systems. Now, “Nothing in Biology Makes Sense Except in the Light of Evolution”
                                    --Theodosius Dobzhansky
(Frolich PowerPoint slide 51)