* Basic genetics (Ch25)

Question 1

Gene

Answer 1

• a short segment of DNA on a chromosome
• controlling the formation of structural / functional protein

Question 2

Mendel’s breeding experiments

Answer 2

• Mendel carried out many breeding experiments on pea plants. He grew and examined a large number of pea plants to collect a large amount of experimental evidence
• Based on his observation on the large number of offspring with different phenotypes, he worked out a possible explanation for the observation by logical deduction

Question 3

Reasons to use garden peas

Answer 3

• small and easy to be cultivated
• grows quickly, life cycle is short
• has bisexual flower

Question 4

Procedures of study

Answer 4

• Remove anthers (prevent self-pollination)
• Cover the flower with plastic bag (prevent further pollination)

Question 5

Mendel’s First Law (Law of Segregation)

Answer 5

• A particular trait is determined by a pair of hereditary factors (alleles) which will segregate during gamete formation, only one factor will go to each gamate
• When two gamete carrying either one factor of a pair fuse, only the dominant factor will be expressed in the phenotype of offspring

Question 6

How Mendel designed his experiment so that his findings were valid and reliable?

Answer 6

1. Studied one or two traits at a time
2. Bred pure line
3. A great number of samples to reduce error
4. Record quantitative results
5. Remove anthers
6. Carefully ruled out controlled variables

Question 7

Monohybrid inheritance (with numbers)

Answer 7

1. Number of offspring of each phenotype
2. Ratio
3. According to Mendel’s First Law
4. Both parents must be heterozygous
   (5. Dominant character
5. Phenotype of parents)

Question 8

Deducing dominant character

Answer 8

父母一樣 子女不同

1. Phenotype of parents -> at least one gene for A
2. Phenotype of offspring -> at least one gene for B
3. Allele of B of offspring must come from either parent
4. At least one parent is heterozygous
5. Heterozygous condition, only dominant allele is expressed, recessive allele is masked
6. Phenotype of both parent -> A must be dominant

Question 9

Blood transfusion -> blood clumping

Answer 9

1. Antigen on red blood cells of donor
2. Antibody in plasma of recipient
3. Antibody act against antigen
4. Blood clumping

Question 10

Why men can donate blood more frequently than female

Answer 10

• Female has periodic blood loss in menstruation

Question 11

50% chance of having a baby boy / girl

Answer 11

• Female produce only one type of egg with X chromosome
• Male produce two types of sperm, carrying X or Y chromosome
• There are equal numbers of the two types of sperms
• Random fertilization

Question 12

Continuous variation

Answer 12

• Variations of certain characteristics that have a wide and continuous range of intermediates between the two extremes
• Normal distribution curve
• Controlled by many genes
• Affected by the environment

Question 13

Discontinuous variation

Answer 13

• Variations that have distinctive categories with no intermediate categories
• Bar chart
• Not easily affected by the environment

Question 14

Why sex-linked disorders are more common in males than females

Answer 14

• Female has two X chromosomes, she has the sex-linked disorder only when both her X chromosomes carry the recessive alleles of the disorder
• She will be normal if she is heterozygous because the recessive allele is masked by the dominant allele
• Male has only one X chromosome, he has the sex-linked disorder if the X chromosome carries the recessive allele

Question 15

Deducing whether a disease is autosomal

Answer 15

Y-linked: 父子（有病 -> 無病）
- All male offspring of a father will be affected individuals
- Son is not affected but father is affected -> not Y-linked

X-linked recessive 父女 / 母子（女有病 -> 男無病）
- An affected female will produce affected male
- Son is not affected but mother is affected -> not X-linked recessive

X-linked dominant 父女 / 母子（男有病 -> 女無病）
- Female offspring of an affected father must be affected (since the father has only one chromosome, he must inherit it to the daughter)
- Daughter is not affected but father is affected -> not X-linked dominant

-> not Y-linked / X-linked recessive / X-linked dominant

Question 16

Deduce whether a disease is X-linked

Answer 16

Not affected (Homozygous dominant) -> Affected
- Affected: 2 diseased alleles, must have one inherited from parents
- Parents no diseased alleles -> not autosomal

-> Sex-linked

Question 17

Mendel’s Second Law

Answer 17

• The hereditary factors assort independently in gamete formation, which leads to variations in gametes, hence variations in F2

Question 18

Expression of phenotypes

Answer 18

• Both parents are pure-bred, the F1 produced are all heterozygous
• In heterozygous condition, dominant alleles will be expressed while the recessive alleles will be masked
  -> Deduce dominant character
• During gamete formation, each allele in a pair of contrasting characters separates independently from the separation of the other pair of contrasting characters
• As F1 individuals are heterozygous with genes for the two characteristics located on different chromosomes, the alleles are assorted independently
• Four types of gametes are produced in F1 generation
• After random fertilization, zygotes with 16 different combinations of genotypes will be produced, leading to expression of 4 genotypes in F2 generation

Question 19

Independent assortment of chromosomes at meiosis

Answer 19

• After pairing of homologous chromosomes, the members of homologous chromosomes pair segregate into the daughter nuclei during first meiotic cell division
• The random segregation of members of homologous chromosome par results in variations between gametes formed, which are of different combinations of alleles
• As the gees for the two characteristics are located on different chromosomes, the alleles for the characteristics are associated independently

Question 20

Ratio of phenotypes

Answer 20

RRYY x rryy: all dominant
RRYy x rryy: 1:1
RrYY x rryy: 1:1
RrYy x rryy: 1:1:1:1

Question 21

Wrinkled pea

Answer 21

• Absence of starch-forming enzyme in the seeds
• Manufacture less starch and store sugar instead
• Lowers water potential of storage cells in the seeds
• More water is taken into these cells from nearby tissues
• Seeds swell and stretch the seed coat as they develop
• Peas become dehydrated at maturity, loss of water causes the wrinkled appearance

Question 22

Fruit flies

Answer 22

• Short life cycle
• Easy to culture