Lecture 13: Human Genetics

Question 1

wild-type traits

Answer 1

• those seen most often in nature
• not necessarily specified by dominant alleles

Question 2

family pedigree

Answer 2

• shows the history of a trait in a family
• allows geneticists to analyze human traits

Question 3

human disorders controlled by a single gene

Answer 3

many human traits
* show simple inheritance patterns
* are controlled by single genes on autosomes
* Sir Archibald Garrod and “inborn errors of metabolism”: first to connect a human disorder with Mendel’s laws of inheritance, suggesting that genetic conditions are caused by defects in specific enzymes

Question 4

carriers

Answer 4

most human genetic disorders are recessive. individuals who have the recessive allele but appear are normal are carriers of disorders

Question 5

cystic fibrosis

Answer 5

• the most common lethal genetic disease in the US
• caused by a recessive allele carried by about 1/31 Americans

Question 6

inbreeding

Answer 6

• prolonged geographic isolation of certain populations can lead to the mating of close relatives
• increases the chance of offspring that are homozygous for a harmful recessive trait

Question 7

achondroplasia

Answer 7

some human genetic disorders are dominant, like achondroplasia, a form of dwarfism
* the homozygous dominant genotype causes death of the embryo
* thus, only heterozygotes have this disorder

Question 8

Huntington’s disease

Answer 8

some human genetic disorders are dominant, like Huntington’s disease, which leads to the degeneration of the nervous system, but does not usually begin until middle age

Question 9

genetic testing

Answer 9

today many tests can detect the presence of disease-causing alleles, most of which are performed during pregnancy
* amniocentesis collects cells from amniotic fluid
* chorionic villus sampling removes cells from placental tissue
* genetic counseling helps patients understand the results and implications of genetic testing

Question 10

incomplete dominance

Answer 10

some patterns of genetic inheritance might appear to violate Mendel’s laws, but they do not when examined closely, like incomplete dominance, in which:
* F1 hybrids have an appearance between the phenotypes of the 2 parents

Question 11

hypercholesterolemia

Answer 11

• a human trait that is an example of incomplete dominance
• is characterized by dangerously high levels of cholesterol in the blood
• heterozygotes have blood cholesterol levels about 2x normal, and
• homozygotes have about 5x normal amount of bloody cholesterol and may have heart attacks as early as age 2

Question 12

ABO blood groups

Answer 12

the ABO blood groups in humans are an example of multiple alleles (and codominance)
* the immune system produces blood proteins called antibodies that bind specifically to foreign carbohydrates
* the carbohydrates are actually polysaccharides located on the surface of red blood cells
* if a donor’s blood cells have a carbohydrate (A or B) that is foreign to the recipient, the blood cells may clump together, potentially killing the recipient
* the clumping reaction is the basis of a blood-typing lab test

Question 13

codominance (in ABO blood groups)

Answer 13

the human blood alleles I^A and I^B are codominant, meaning that both alleles are expressed in heterozygous individuals who have type AB blood

Question 14

pleiotropy

Answer 14

when 1 gene influences several characters
* i.e. sickle-cell disease

Question 15

sickle-cell disease

Answer 15

• exhibits pleiotropy
• results in abnormal hemoglobin proteins
• causes disk-shaped red blood cells to deform into a sickle shape with jagged edges

Question 16

polygenic inheritance

Answer 16

the single additive effects of 2 or more genes on a single phenotype

Question 17

multifactorial inheritance

Answer 17

the interaction of multiple genes and environmental factors on a single phenotype
* several types of human epilepsy are multifactorial

Question 18

the role of environment

Answer 18

• many human characters result from a combination of heredity and environment
• only genetic influences are inherited
• one of Mendel’s characters was flowering time, which he did not present the data for in his original 1866 paper. however, the environment (moisture, seed depth, temperature) significantly influenced this trait

Question 19

sex chromosomes

Answer 19

sex chromosomes influence the inheritance of certain traits. for example, humans have a pair of sex chromosomes designated
* X and Y: male
* X and X: female

Question 20

sex-linked gene

Answer 20

any gene located on a sex chromosome is called a sex-linked gene
* most sex-linked genes are found on the X chromosome. some rare genes are on the Y
* e.g. red-green colorblindness
* e.g. hemophilia

Question 21

red-green colorblindness

Answer 21

• a common human sex-linked disorder
• caused by a malfunction of light-sensitive cells in the eyes

Question 22

hemophilia

Answer 22

• a sex-linked recessive blood-clotting trait that may result in excessive bleeding and death after relatively minor cuts and bruises
• European royal families are heavily affected