Chapter 4: Modification of Mendelian Ratios

Question 1

Name what each of the following symbols mean:

1) +/+
2) +/e
3) e/e
4) e^+/e^+
5) e^+/e

Answer 1

1) +/+ homozygous wild type
2) +/e heterzygous (wild type gene is usually dominant and therefore expressed)
3) e/e homozygous mutant
4) e^+/e^+ homozygous wild type
5) e^+/e heterozygous

Question 2

distinguish between loss of function mutation (null mutation), gain of function mutation, and neutral mutation

Answer 2

loss/null: protein loses its function d/t altered structure

gain: more protein is expressed
neutral: no change in protein function

Question 3

what is incomplete dominance?

Answer 3

neither allele expresses dominance over the other. If combined, they will express an intermediate/blended phenotype
i.e. red flower x white flower = pink flower
note that if 2 pink flowers are crossed, offspring will be 1/4 red, 1/4 white, and 1/2 pink (phenotypic ratio is identical to genotypic ratio)

Question 4

What is the genotype ratio for incomplete dominance when 2 of the heterozygotes are crossed

Answer 4

1:2:1 ratio
1 = homozygous for one trait
2 - heterozygotes
1 = homozygous for the other trait

Question 5

What is codominance?

Answer 5

the joint expression of the gene product of both alleles in a heterozygote
i.e. If a person gets an allele for glycoprotein M from mom and an allele for glycoprotein N from dad, the child will express both types of glycoproteins

Question 6

What is the genotypic ratio in codominance when 2 heterozygotes are crossed?

Answer 6

1:2:1 ratio
1 = homozygous for one trait
2 - heterozygotes
1 = homozygous for the other trait

Question 7

What is multiple alleles?

Answer 7

where there are more than 2 alleles of a single locus that produce fully functional but distinct products
i.e. blood type A, B, and O
blood type A expresses A antigen, B expresses B antigen, and O expresses no antigen

Question 8

What is the Bombay phenotype?

Answer 8

in the example of blood types: there is a mutation that prevents the H substance (that is required to add the A and/or B antigen sugar groups to the fatty acid protruding from the membrane of the RBC) from being completely formed, thus even though the individual has an A and/or B allele, there will be no A and/or B antigen expressed

Question 9

What is H substance?

Answer 9

a precursor molecule that attaches the A and/or B antigen sugar molecule to the lipidmolecules protruding from the RBC membrane.

Question 10

Which type of serum antibodies will an individual produce if that individual has an O blood type?

Answer 10

anti-A and anti-B antibodies (because type O blood does not express either antigen and will see each antigen as foregin)

Question 11

What is the recessive lethal allele?

Answer 11

refers to a mutant recessive allele in a gene product that is necessary for survival and if an offspring inherits 2 of these mutant alleles, that particular gene product is not functional and the offspring will die (the death of which is determined by when that gene product is essential for survival)

Question 12

The agouti color in mice is considered dominant and the yellow color recessive. The recessive allele is lethal if an offspring has 2 recessive alleles. However, with respect to phenotype, the yellow color is dominant. If 2 yellow mice are crossed, what will be the ratio of the offspring?

Answer 12

2/3 yellow and 1/3 agouti
the reason for the ratio being in thirds is because not all will survive: in development, 1/4 are agouti, 1/2 are yellow heterozygotes, and 1/4 are homozygote recessive; the 1/4 of the mice that inherited the 2 recessive alleles died and are not considered in the ratio of the offspring listed above

Question 13

what is epistasis?

Answer 13

occurs when the expression of one gene or gene pair masks or modifies the expression of another gene or gene pair
i.e. homozygous recessive allele at one locus (X) prevents expression of the alleles at a second locus (Y). The Y gene is hypostatic to X and X is epistatic to Y

Question 14

If the ratios are expressed in 16ths, it indicates that there are _1__ genes involved and are interacting during expression of the __2___, even though only a single character was followed

Answer 14

1) 2 genes

2) phenotype

Question 15

The phenotypical ratio among the progenies of a genetic cross is found to be 9:7. Which of the following statement is likely true?
A) the phenotype is controlled by another gene
B) the phenotype is controlled by 2 genes
C) the cross follows the rules of Mendelian genetics

Answer 15

B

Question 16

What is the phenotypical ratio for 2 gene interactions?

Answer 16

9:3:4
*Note these add up to 16
9 will phenotypically dominant (i.e. 9 agouti mice)
3 will be recessively dominant (3 black mice)
4 will have masked/absence of the expression (i.e. 4 albino mice)

Question 17

What is dominant epistasis?

Answer 17

refers to when only one dominant allele is needed at the first locus results in a particular expression regardless of the genotype at the second locus i.e. dominant AA/Aa allele results in white squash regardless if the genotype at the second locus is BB/Bb (yellow) or bb (green). The only way the squash can be yellow or green is if the offspring has both recessive alleles aa

Question 18

If 2 white heterozygous squash are crossed, with the white Aa being a dominant epistasis allele, what phenotypic ratio will the offspring be? (Assume gene at second locus is BB/Bb for yellow and bb for green)

Answer 18

12 white: 3 yellow: 1 green

9/16 = AB white and 3/16 = Abb white; 3/16 = aaB yellow and 1/16 = aabb green

Question 19

All cases of the modified Dihybrid ratios have two things in common:

1) principles of ______ and ______ are still followed
2) F2 ratios are expressed in the 16ths-suggests ____ gene pairs are involved

Answer 19

1) segregation and independent assortment

Question 20

What is complementary gene interaction?

Answer 20

in the example of white and purple flowers where AAbb & aaBB flowers are white, but when crossed produce a ratio of 9:7 purple:white flowers: the presence of at least one dominant allele of each two gene pairs is essential for flowers to be purple

Question 21

What is hemizygous?

Answer 21

• refers to the male sex chromosomes of XY, for the Y chromosome only contains a few genes that are NOT found on the X chromosome and vice versa
• males cannot by homozygous or heterozygous for X-linked genes

Question 22

What is an X-linked trait?

Answer 22

the transmission and expression of the genes located on the X chromosome

• have a unique pattern of inheritance
• unlike the outcome of the typical monohybrid cross, reciprocal crosses between white and red-eyed flies did not yield identical results; hence, phenotypic ratios in both F1 and F2 generations are dependent on whether or not the P1 white-eye parent was male or female

Question 23

What is the criss-cross pattern of inheritance?

Answer 23

whereby phenotypic traits controlled by recessive X-linked genes are passed from homozygous mothers to all sons

Question 24

What is penetrance?

Answer 24

refers to a percentage of individuals who show at least some degree of expression of a mutant genotype
i.e. the phenotypic expression of many mutant alleles in fruit flies can overlap with wild type. If 15% of mutant flies show the wild type appearance, the mutant gene is said to have a penetrance of 85%

Question 25

What is expressivity?

Answer 25

reflects the range of expression of the mutant genotype
i.e. flies homozygous for the recessive mutant eyeless gene yield phenotypes that range from the presence of normal eyes to partial reduction in size to the complete absence of one or both eyes; hence, the expressivity ranges from complete loss of both eyes to completely normal eyes

Question 26

Give an example of a late-onset genetic disease

Answer 26

Huntington’s disease: caused by an autosomal dominant mutation; average onset is at age 38. Thus, if a person doesn’t know they have it, they can reproduce and have a 50% chance of passing on the mutation to their offspring.

Question 27

What is genetic anticipation?

Answer 27

refers to the heritable disorders that exhibit a progressively earlier age of onset and an increased severity of the disorder in each successive generation

Question 28

give an example of a genetic anticipation disorder

Answer 28

myotonic dystrophy: an autosomal dominant disorder; mildly affected individuals develop cataracts as adults but have little or no muscular weakness. severe affected individuals demonstrate more extensive myopathy and may be intellectually disabled

Question 29

What is extranuclear inheritance?

Answer 29

refers to an organism’s phenotype being affected by the expression of genes that are not in the nucleus; contains 2 broad categories:

• phenotype is affected by expression of genes contained in the DNA of mitochondria or chloroplasts rather than the nucleus (aka organelle heredity)
• maternal effect: organism’s phenotype is determined by the genetic information expressed in the gamete of the mother-such that, following fertilization, the developing zygote’s phenotype is influenced by the gene products directed by the genotype of the mother

Question 30

Many disorders in humans are known to be due to mutations in mitochondrial genes. For example, myoclonic epilepsy and ragged-red fiber disease (MERF) demonstrates a pattern of inheritance consistent with ______ transmission. Only offspring of affected mothers inherit this disorder, while the offspring of affected fathers are normal.

Answer 30

maternal

The disease is named for the presence of “ragged-red” skeletal muscle fibers that exhibit blotchy red pathces resulting from the proliferation of aberrant mitochondria (which are only inherited from the mother)

Question 31

What is heteroplasmy?

Answer 31

Refers to mutant conditions in organelles (i.e. mitochondria) not being revealed because there are also normal organelles present that exhibit the wild type function and therefore, help to mask the mutation