Chapter 20
Genes and Medical Genetics
1. Recognize the conventions biologists use to represent alleles using letters. [20.1, p.404, Fig. 20.2]
2. Distinguish between phenotype and genotype, dominant and recessive, and homozygous and heterozygous. [20.1, p.404]
3. Understand how gametes carry only one gene for each trait. [20.2, po. 405, Fig. 20.3]
4. List a variety of simple recessive and dominant traits in humans. [20.2, p. 405, Fig. 20.4]
5. Solve one-trait autosomal genetics problems using a Punnett square. [20.2, p. 406, Fig. 20.5-20.6]
6. Describe disorders that are inherited as recessive genes. [20. 2, p.407, Fig. 20.7]
7. Describe disorders that are inherited as a dominant gene. [20.2, p.408, Fig. 20.8]
8. Explain how to interpret pedigree charts. [20.2, p.409, Fig. 20.9, 20.10, 20.15]
9. Discuss one trait inherited as a polygenic trait. [20.3, p.411, Fig. 20.11]
10. Explain what is meant by multiple alleles and give an example. [20.3, p.412, Fig, 20.12]
11. Explain what is meant by incomplete dominance and give an example. [20.4, p.413, Fig. 20.13]
12. Show how sex-linked traits are generally inherited on the X chromosome. [20.4, p.414, Fig. 20.14]
13. Discuss several sex-linked disorders. [20.4, pp.415-416, Figs. 20.15-20.16]
14. Distinguish between sex-linked and sex-influenced traits. [20.4, pp.415-418]
15. Understand and use the bold-faced and italicized terms included in this chapter. [Understanding Key Terms, p.419]
The genotype represents the
actual genes of an individual. Alleles are alternate forms of a gene at the
same gene locus. Individuals can be homozygous dominant (EE) for a trait,
heterozygous (Ee), or homozygous recessive (ee). The phenotype refers to the outward expression of the
genotype (e.g., red hair).
Forming the Gametes
During oogenesis and spermatogenesis (collectively, gametogenesis), chromosome number is reduced to half, and each gene pair for a trait is separated, so that the offspring receives one gene for each trait from each parent.
Figuring the Odds
A parent who is homozygous dominant or homozygous recessive for a trait can pass on only one type of gamete in each case. Heterozygous parents can pass on either the dominant or the recessive gene for a given trait. A Punnett square is useful for determining the possible outcomes of a genetic cross. A cross of two individuals who are both homozygous for a trait results in a 100% chance of having an offspring who is homozygous for the trait.
A cross of two heterozygotes results in a 25% chance that the offspring will be homozygous recessive, a 50% chance that the offspring will be heterozygous, and a 25% chance that the offspring will be homozygous dominant. Each offspring has a 25% chance of being homozygous recessive or of being homozygous dominant.
Recessive Disorders
Tay-Sachs Disease
Tay-Sachs disease is inherited as an autosomal recessive. Between four and six months of age, an affected infant shows neurological impairment. The child gradually becomes blind, helpless, and paralyzed, and usually dies by age four. Tay-Sachs results from a lack of hexosaminidase A and the storage of its substrate, glycosphingolipid in lysosomes. This disease is most prevalent in Jewish people from central and eastern European descent.
Cystic Fibrosis
Cystic fibrosis is the most common lethal genetic disease among U.S. Caucasians. The thick mucus in bronchial passageways and pancreatic ducts interferes with the functioning of these organs. The defect lies in a chloride ion transport protein within plasma membranes. When chloride passes through, water normally follows. In cystic fibrosis patients, a lack of water following through results in the thick mucus. The gene for the defect is on chromosome 7.
Phenylketonuria (PKU)
Individuals
with phenylketonuria lack an enzyme needed for the
normal metabolism of phenylalanine. Phenylketone thus
accumulates in the urine. If the infant is not put on a
phenylalanine-restrictive diet in infancy until age seven, brain damage and
severe mental retardation result.
Dominant Disorders
Neurofibromatosis
Neurofibromatosis (von Recklinghausen disease) is inherited as an autosomal dominant. People with this condition develop benign neurofibromas under the skin and in various organs. The effects can range from mild to severe, and some neurological impairment if possible. The gene for this trait is a nested gene on chromosome 17.
Pedigree Charts
Pedigree charts are a means of constructing a family tree and indicate which individuals are affected by a trait. Since recessive and dominant traits exhibit different patterns of inheritance, pattern of inheritance can be partially determined by examining a pedigree chart. Carriers are normal individuals capable of producing affected children.
Polygenic
Inheritance
Polygenic traits are those governed by more than one gene pair. Several pairs of genes may be involved in determining phenotype.
Skin Color
The inheritance of skin color, determined by an unknown number of gene pairs, is a classic example of polygenic inheritance.
Polygenic Disorders
Many human traits, like
allergies, schizophrenia, cleft lip, and hypertension, among others, appear to
be inherited as polygenic traits. The expression of some genes is subject to
environmental influences (e.g., a child allergic to ragweed will never express
that trait while living in the
In multiple alleles, more than two alternative types exist for a gene pair. ABO blood grouping is an example.
ABO Blood Types
The ABO blood grouping represents surface marker proteins on red blood cells. A person can have a gene for an A marker or a B marker, which are codominant, or lack an A or B marker, designated type O, which is recessive.
Human ABO blood types can then be type A (which can be AA or AO), type B (BB or BO), type AB (AB), or type O (OO).
Patterns of dominance often go beyond simple dominant or recessive traits. Codominance means that both alleles are expressed (type AB blood). Incomplete dominance is exhibited when the heterozygote shows not the dominant trait but an intermediate phenotype, representing a sort of blending of traits (e.g., skin color or hair type).
Sickle-Cell Disease
Sickle-cell disease is an example
of incomplete dominance. An individual with two genes for normal hemoglobin has
normal hemoglobin. A heterozygote has a normal gene and a gene for sickled hemoglobin. An individual with two sickling genes has sickle-cell disease. What may have
maintained this apparently detrimental gene in equatorial
Sex-linked traits are genes (traits) carried most frequently on the X chromosome. (The Y chromosome is too small.)
X-Linked Alleles
In X-linked traits, the gene is carried on the X chromosome. Since males have only one copy of the X chromosome, they show the phenotype for the allele they possess and are thus much more likely than females to show a recessive trait. A female must have two copies of a recessive trait (one on each X chromosome) to display it. If a female has only one copy of a recessive gene, she is said to be a carrier and will pass the trait on to 50% of her sons, on average.
X-Linked Disorders
Color Blindness
Three types of cones are in the retina: those that detect red, those that detect green, and those that detect blue. Genes for blue cones are autosomal; those for red and green cones are on the X chromosome. Males are much more likely to have red/green colorblindness than are females.
Muscular Dystrophy
Duchenne muscular dystrophy is X-linked and is characterized by progressive muscle deterioration during childhood. The absence of the protein dystrophin causes the disorder.
Hemophilia
Hemophilia (bleeder’s disease)
can be traced to Queen
Sex-Influenced Traits
Some traits carried on autosomes such as male-pattern baldness, can be influenced by gender. In this instance, the male hormone testosterone is the culprit.
Genetics of Your Students
1. Certain obvious human traits can demonstrate the idea of dominant and recessive traits, and can even be discussed in the greater context of population genetics. Canvas your class to see how many fall into each of the following categories:
|
Expression of Dominant
Trait |
Expression of Recessive
Trait |
|
Dark hair color |
Light or red hair color |
|
Farsightedness |
Normal vision |
|
Freckles |
No freckles |
|
Dark eyes |
Blue eyes |
|
Free earlobes |
Attached earlobes |
|
Six fingers or toes |
Five fingers or toes |
|
Cheek dimples |
Lack of dimples |
|
Full lips |
Thin lips |
|
Widow’s peak |
Straight hair line |
|
Tongue roller |
Cannot roll tongue |