Lecture 23

Question 1

Define Sexual Reproduction:

Answer 1

2 individuals combining genetic information resulting in a progeny with a new genetic combination that is different from both parents.

Question 2

Define Asexual reproduction:

Answer 2

reproduction without genetic exchange. The resulting progeny is genetically identical to the parents.

Question 3

Define Sexual Phenotypes:

Answer 3

Male and female morphology - a results of sexual differentiation.

Question 4

Define Isogamy: Vs. Define Anisogamy

Answer 4

Isogamy: Involved fusion of two gametes, which are identical in size and form.
eg. yeast mating

Anisogamy: Involved fusion of two gametes, which differ in size and/or form.
The larger gametes are from female and the smaller gametes are from the male
eg. oogamy

Question 5

What is Sex? How to Determine it?

Answer 5

“Sex” in this context = sexual phenotype
Sex may be determined:
- chromosomally
- genetically
- environmentally

Question 6

Define hermaphroditism

Answer 6

Both sexes present in the same individual (eg. bisexual flowers have both male and female parts)

Question 7

Monoecious species vs Dioecious species and examples:

Answer 7

Monoecious species = have both male and female reproductive parts on same individual

Dioecious species = individuals have either male or female reproductive parts
example: Humans

Sex may be determined:
- chromosomally
- genetically
- environmentally

Question 8

How to Determine Sex: Chromosomal
Autosomal Vs Sex Chromosomes (2)
Sex systems?
XX-XY, ZZ-ZW

Answer 8

Autosome = chromosomes considered in Mendel’s Laws of Independent Assortment and Equal Segregation.

Sex Chromosomes:
1. most animals and many plants have a special pair of chromosomes that determine the sex

2. Segregate equally BUT phenotypic ratios of the progeny often differ from autosomal ratios.

• XX-Y and ZZZW systems

XX-XO system of insects
- Females have 2 X chromosomes (XX)
- males have a single X and no other sex chromosome (XO)

Question 9

Homogametic vs. Heterogametic vs. Hemigametic Sex

Answer 9

Homogametic sex = produces gametes of one type
- females in XX -XY and XX-XO systems: males in ZZ-ZW systems

Heterogametic sex = produces gametes of more than one type
- males in XX-XY systems; females in ZZ-ZW systems

Hemigametic sex = males in XX-XO systems

Question 10

Chromosomal Sex in Humans; explain how many, what and where gene? (3)

Answer 10

1. Humans = 22 homologous pairs of autosomes and 2 Sex chromosomes, X and Y
2. Females = XX, Males = XY
3. Presence of the sex-determining region Y (SRY) gene on the Y Chromosome determines maleness

Question 11

Genic Sex Determining Systems:
found/determined where, Chromosomes, WHO? (6)

Answer 11

1. Genes at one or more loci determine sex of an individual - as for chromosomal sex determination systems HOWEVER
2. No Obvious differences in the Chromosomes of Males and Females
3. No SEX CHROMOSOMES
4. Some Protozoa
5. Some Plants
6. Some Fungi

Question 12

How does Environment Sex Determine?
(TSD) - 3 EXAMPLES

Answer 12

1. Temperature Sex Determination (TSD)
   - Sexual phenotype is affected by temperature during embryonic development
   - response differs among organisms

EXAMPLE:

1. Australian Brush turkey: equal males and females at a next temperature of 34 degrees Celsius; more males when cooler and more females when warmer
2. American Alligator: nearly 100 per cent males at a nest temperature of 33 degrees Celsius; 95 per cent females when slightly warmer
3. Turtles: warm nest temperatures produce females and cooler produce males,

Question 13

Define Sex Linkage

Answer 13

Sex Linkage - characteristics determined by genes on sex chromosomes
- an extension of Mendel’s laws

Question 14

Drosophila; SEX characteristics, chromosome, Eye colour, Morgan’s cross, F1, F2 Cross, results

Answer 14

1. Drosophila
   - 3 pairs of autosomes
   - 1 pair of sex chromosomes
   - X and Y with females, XY= males
2. Thomas Hunt Morgan
   - found an eye colour mutation in Drosophila that was defined by a gene:
   - on a recognisable chromosome
   - with a pattern of transmission reflecting that of the chromosome during Reproduction
3. Morgan’s cross:
   red-eyed female X white-eyed
   male w+ allele * w allele

F1 all red-eyed
w+ (w = recessive)

Crossed F1 males and females

F2 3:1, red-eyed to white-eyed
2 red-eyed females 1 red-eyed male
1 white-eyed male *all the white-eyed flies are male

4. Explanation of inheritance patterns:

All F1 female flies (w+/w heterozygous wild-type)
*received w+ from mother
*received w from father

All F1 male flies (w+ hemizygous)
*received w+ from mother
*Y from father only

In the F2:
*w from the F1 female is passed to 1⁄2 the daughters
and 1⁄2 the sons

*BUT only the males express it (females with w also have w+)

Question 15

Explain the Reciprocal Cross of Drosophila:
F1, F2 GEN, RESULTS

Answer 15

The reciprocal cross:
All F1 female flies (w+/w heterozygous wild-type)
*received w+ from father
*received w from mother

All F1 male flies (w hemizygous) *received w from mother *received Y from father

2. In the F2:
   *w from the F1 male is passed to all the daughters
   *w from F1 female is passed to 1⁄2 the daughters and 1⁄2 the sons

Resulting in:
1⁄2 the progeny = white-eyed (1⁄2 female, 1⁄2 male) 1⁄2 the progeny = red-eyed (1⁄2 female, 1⁄2 male)

Results
Some red-eyed males some white-eyed females
= exceptions

Exceptions found in every cross (too common to be spontaneous mutations)

Question 16

How Was Morgan’s Experiment Extended? How was “exceptions” proof of chromosome theory?
(2)

Answer 16

Calvin Bridges *Morgan’s student

1. *secured proof of the chromosome theory by showing “exceptions” were a result of chromosome behaviour during meiosis
2. *examined many progeny of white-eyed female x red-eyed male cross
   *nearly all red-eyed females and white-eyed males
   *exceptions were white-eyed females and red-eyed males

Question 17

What is Nondisjunction? When Does it occur? Causes (3), In who?

Answer 17

1. Nondisjunction of the X chromosomes in females during meiosis and gamete formation
   *eggs with 2 X chromosomes
   *eggs with no X chromosomes (nullo-X)
2. Fertilisation of eggs with normal sperm *zygotes with abnormal sex chromosomes (“exceptional” males and females)
   *nonviable progeny
3. Bridges confirmed the results by examining
   chromosomes of progeny
4. Cause for nondisjunction not known, perhaps *faulty chromosome movement, or
   *imprecise or incomplete pairing, or *centromere malfunction
5. May also have nondisjunction in males (both males and females)

Question 18

Explain Sex Linkage; for example, where does it occur?, how does it duffer from autosomal inheritance patterns?

Answer 18

1. Example: eye colour in Drosophila is X-linked
2. *there are also Y-linked inheritance patterns; however, many fewer due to less
   genetic information on the Y chromosome
3. *sex-linked inheritance patterns contrast with autosomal inheritance patterns
   *see different ratios of traits in different sexes *see differences in reciprocal crosses

Question 19

X-linked Recessive Disorders: Red-Green Colour Blindness (3), What and How it occur?

Answer 19

1. *light is perceived due to light-absorbing receptors in the eye
2. *genes for the green and red light receptors are on the X chromosome (gene for the
   the blue light receptor is autosomal)
3. *individuals with red-green colour blindness have a mutation in one of the two genes on the X chromosome

Notice:
*different ratios of traits in different sexes
*different ratios of affected individuals in the reciprocal cross (b) compared to the cross in (a)

Ishihara colour blindness chart
*red-green colour-blind individuals see a 3
*individuals with normal colour vision see an 8
*affected female (homozygote) passes the trait to all her male progeny; female progeny are not affected, but are carriers (heterozygotes)

Question 20

Explain Human X-Linked Recessive Disorders. Frequency? Who affected? How Continued in the generations?

Answer 20

1. Characteristics
   . *relatively rare
2. *many more males than females show the phenotype
3. *none of the progeny of an affected male will show the phenotype,
   *and none of his sons will pass the condition to their progeny
   *however, all his daughters will be carriers (heterozygous) and their sons will be affected

Question 21

Example of X-Linked Recessive Disorders: Haemophilia A; what is it?

Answer 21

1. abnormal or lack of blood clotting Factor VIII
2. excessive bleeding, spontaneous bleeding in joints

Question 22

Human Pedigree Analyses: X-linked Dominant Disorders - Characteristics (2), Example (Hypophosphatemia)

Answer 22

Characteristics
*all daughters of affected males will have the condition
*half the sons and daughters of affected heterozygous females and unaffected males will have the condition

An example: vitamin D-resistant rickets (hypophosphatemia)
*low phosphate in the blood due to high excretion in the urine
*reduced mineral deposition, leading to bone deformities, stiff joints

Question 23

Human Pedigree Analyses: Y-linked
Characteristics (3)

Answer 23

Characteristics
*males transmit genes to their sons
*not common as not many genes are on Y chromosome
*SRY gene is in the differential region of the Y chromosome – so maleness is Y-linked

Question 24

Sex-Influenced Characteristics: WHAT OR HOW IS IT? inherited?
2 EXAMPLES: determined by? Recessive or Dominant in males/females?

Answer 24

1. *controlled by autosomal genes
2. *inherited according to Mendel’s Laws, except they are more readily expressed in
   one sex

EXAMPLE 1
*e.g. human pattern baldness
*determined by autosomal gene B
*dominant in males *recessive in females

EXAMPLE 2
*e.g. beards on goats
*determined by autosomal gene Bb
*dominant in males *recessive in females

Question 25

What are Sex-Limited Characteristics (3 and example) autosomal?

Answer 25

1. *an extreme form of sex-influenced inheritance
2. *controlled by autosomal genes
3. *inherited according to Mendel’s Laws, except they are expressed in only one sex

e.g. plumage pattern in domestic chickens called cock feathering *autosomal recessive trait sex-limited to males