Mendelian Genetics

Question 1

Mendel’s experiment

Answer 1

1. In 1857, GregorMendel worked out the rules of inheritance through a series of brilliant experiments on garden peas.
2. Early in the 20th century, Sutton and Boveri formulated the chromosome theory of inheritance:-which proposes that meiosis causes the patterns of inheritance that Mendel observed

Question 2

What questions was Mendel trying to answer?

Answer 2

1. Mendel was addressing the basic question of;
   - Why offspring resemble their parents?
   - How transmission of traits occurs?
2. During his time two hypotheses were formulated to answer these questions;
   - Blending inheritance
   - Inheritance of acquired characters

Question 3

Mendel’s experimen

Answer 3

1. Mendel used a Scientific Approach:
   ─Experimental and Quantitative
2. Identified to laws of inheritance
   -Law of Segregation
   -Principle of Independent Assortment
3. Mendel documented a particulate Inheritance through his experiments with garden peas.
4. The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes).

Question 4

Mendel’s model plant

Answer 4

Mendel chose a common garden pea (Pisumsativum) as his model organism because:

• It is easy to grow
• Is inexpensive
• Has a short reproductive cycle
• he could control matings
• The traits he studied were easily recognizable

Question 5

trait

Answer 5

any physical characteristic of an individual.

Question 6

Heredity

Answer 6

the transmission of traits from parents to their offspring

Question 7

Inheritance of a single trait

Answer 7

His first experiments involved crossing pure lines that differed in just one trait.
-The adults in the cross were the parental generation, their offspring the F1 generation (for “first filial”).

Question 8

Results: Certain Traits “Recede”

Answer 8

1. In a cross between plants with purple flowers (PP) and white flowers (pp), all of the F1 progeny had purple flowers (Pp).
2. The white flower phenotype receded.
3. The results of a reciprocal cross were identical.
   - reciprocal cross = Inverse cross

Question 9

The Principle of Segregation

Answer 9

1. Explains the 3:1 ratio:
   - Alleles segregate into different gametes during egg and sperm formation, then come back together when an egg is fertilized by a sperm to form a zygote.
2. Two copies of the same allele (AA or aa) are homozygous –pure lines.
   - Different copies (Aa) are heterozygous.
   - A mating between parents that are heterozygous for a single trait is a monohybrid cross.

Question 10

Testing the model

Answer 10

Mendel used self-crosseswith the F2 progenyto test his model.
-The results (F3) were as predicted.-Plants with wrinkled seeds produced only offspring with wrinkled seeds.-Plants with round seeds produced offspring with round or wrinkled seedsin the expected ratio

Question 11

Mendel used dihybrid crosses to

Answer 11

determine whether the principle of segregation holds true if parents differ in more than one trait.

Question 12

Dihybrid cross

Answer 12

mating between parents that are both heterozygous for two traits

Question 13

The principle of independent assortment,

Answer 13

which states that alleles of different genes are transmitted independently of each other

1. F2 phenotype ratio from dihybrid crosses supports the principle of independent assortment.
2. This law appliesonly to genes on different non-homologous chromosomes or those far apart on the same chromosome.
3. Genes located near each other on the same chromosome tend to be inherited together (Linked genes).

Question 14

Probability laws govern Mendel’s model

Answer 14

1. Mendel’s laws of segregation and independent assortment reflect the rules of probability.
2. When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss.
3. In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles.

Question 15

Mendel’s Contributions to the study of heredity

Answer 15

1. Described the basic rules of transmission genetics:-the patterns that occur as alleles pass from one generation to the next.
2. Chromosome theory of inheritance arose out of the careful observations of meiosis. -Mendel’s rules can be explained by independent alignment and separation of homologous chromosomes at meiosis I.

Question 16

incomplete dominance

Answer 16

the heterozygotes have an intermediate phenotype

*Ratio: 1:2:1

Question 17

codominance

Answer 17

Neither allele is dominant or recessive

Question 18

Pleiotropy

Answer 18

one gene, many phenotypic effects-E.g. Hereditary diseases have multiple symptoms (sickle cell)

Question 19

epistasis

Answer 19

a gene at one locus alters the phenotypic expression of a gene at a second locus-E.g. in many mammals coat color depends on two genes

Question 20

discrete traits (qualitatively difference).

Answer 20

E.g. seed color is either yellow or green -no intermediate phenotypes exist.

Question 21

quantitative traits

Answer 21

Traits that are not discrete but instead fall into a continuum

Question 22

polygenic

Answer 22

These traits are produced by the actions of many genes

Question 23

Principle of independent assortment

Answer 23

1. F2 phenotype ratio from dihybrid crosses supports the principle of independent assortment.
2. This law appliesonly to genes on different non-homologous chromosomes or those far apart on the same chromosome.
3. Genes located near each other on the same chromosome tend to be inherited together (Linked genes)

Question 24

Probability laws govern Mendel’s model

Answer 24

1. Mendel’s laws of segregation and independent assortment reflect the rules of probability.
2. When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss.
3. In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles
4. The Multiplication andAddition Rules also apply. 5. The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities

Question 25

Autosomal recessive traits

Answer 25

1. If a phenotype results from an autosomal recessive allele, the individuals with the trait must be homozygousand the parents are heterozygous carriers.
   E.g.Sickle-Cell Disease and Cystic fibrosis

Question 26

Autosomal dominant traits

Answer 26

1. If a phenotype results from an autosomal dominant allele, the individuals with the trait are either homozygousor heterozygous dominant
   E.g.Achondroplasia

Question 27

Linked Genes

Answer 27

1. Linked genes deviate from the principle of independent assortment.
2. This law applies only to genes on different non-homologous chromosomes or those far apart on the same chromosome.
3. Genes located near each other on the same chromosome tend to be inherited together (Linked genes) because of genetic recombination

Question 28

Discovery of the chromosomes

Answer 28

1. Fruit flies (Drosophila melanogaster) became a model organism for genetic research early in the 20th century
2. Thomas Hunt Morgan mated female flies with red eyes with a male with whites eye color.
3. F1 generation had red eyes –he concluded that the white eye phenotype color was recessive.
4. In the F2 generation, only males had white eye colo

Question 29

Sex-linked inheritance

Answer 29

1. Thomas Hunt Morgan proposed that:
   - The gene for white eye color in fruit flies resides on the X-chromosome.
   - The Y-chromosome does not carry an allele for this gene. -This is the hypothesis of sex-linked inheritance (or X-linkage).
   - If a particular gene for a disorder resides in the Y-chromosome, then it is Y-linkage.

Question 30

Sex-linked human disorders

Answer 30

1. Many male will have the phenotypes than females.
2. Sons get the allele from their mothers.
3. The father cannot pass the allele to his son, bit his daughters will be carriers.
   - Examples of X-linked diseases:
   - Duchene Muscular disorder –progressive weakening of muscles and loss of coordination.
   - Hemophilia-lack of one or more proteins required to form blood clots.

Question 31

X-inactivation in females mammals

Answer 31

1. One of the X chromosomes in mammals is inactivated.
2. The inactive chromosome condenses to form a structure called a bar body
3. Most of the genes in the bar body are not expressed.