Lecture 23 Flashcards
Define Sexual Reproduction:
2 individuals combining genetic information resulting in a progeny with a new genetic combination that is different from both parents.
Define Asexual reproduction:
reproduction without genetic exchange. The resulting progeny is genetically identical to the parents.
Define Sexual Phenotypes:
Male and female morphology - a results of sexual differentiation.
Define Isogamy: Vs. Define Anisogamy
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
What is Sex? How to Determine it?
“Sex” in this context = sexual phenotype
Sex may be determined:
- chromosomally
- genetically
- environmentally
Define hermaphroditism
Both sexes present in the same individual (eg. bisexual flowers have both male and female parts)
Monoecious species vs Dioecious species and examples:
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
How to Determine Sex: Chromosomal
Autosomal Vs Sex Chromosomes (2)
Sex systems?
XX-XY, ZZ-ZW
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
- 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)
Homogametic vs. Heterogametic vs. Hemigametic Sex
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
Chromosomal Sex in Humans; explain how many, what and where gene? (3)
- Humans = 22 homologous pairs of autosomes and 2 Sex chromosomes, X and Y
- Females = XX, Males = XY
- Presence of the sex-determining region Y (SRY) gene on the Y Chromosome determines maleness
Genic Sex Determining Systems:
found/determined where, Chromosomes, WHO? (6)
- Genes at one or more loci determine sex of an individual - as for chromosomal sex determination systems HOWEVER
- No Obvious differences in the Chromosomes of Males and Females
- No SEX CHROMOSOMES
- Some Protozoa
- Some Plants
- Some Fungi
How does Environment Sex Determine?
(TSD) - 3 EXAMPLES
- Temperature Sex Determination (TSD)
- Sexual phenotype is affected by temperature during embryonic development
- response differs among organisms
EXAMPLE:
- Australian Brush turkey: equal males and females at a next temperature of 34 degrees Celsius; more males when cooler and more females when warmer
- American Alligator: nearly 100 per cent males at a nest temperature of 33 degrees Celsius; 95 per cent females when slightly warmer
- Turtles: warm nest temperatures produce females and cooler produce males,
Define Sex Linkage
Sex Linkage - characteristics determined by genes on sex chromosomes
- an extension of Mendel’s laws
Drosophila; SEX characteristics, chromosome, Eye colour, Morgan’s cross, F1, F2 Cross, results
- Drosophila
- 3 pairs of autosomes
- 1 pair of sex chromosomes
- X and Y with females, XY= males - 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 - 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
- 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+)
Explain the Reciprocal Cross of Drosophila:
F1, F2 GEN, RESULTS
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
- 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)
How Was Morgan’s Experiment Extended? How was “exceptions” proof of chromosome theory?
(2)
Calvin Bridges *Morgan’s student
- *secured proof of the chromosome theory by showing “exceptions” were a result of chromosome behaviour during meiosis
- *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
What is Nondisjunction? When Does it occur? Causes (3), In who?
- Nondisjunction of the X chromosomes in females during meiosis and gamete formation
*eggs with 2 X chromosomes
*eggs with no X chromosomes (nullo-X) - Fertilisation of eggs with normal sperm *zygotes with abnormal sex chromosomes (“exceptional” males and females)
*nonviable progeny - Bridges confirmed the results by examining
chromosomes of progeny - Cause for nondisjunction not known, perhaps *faulty chromosome movement, or
*imprecise or incomplete pairing, or *centromere malfunction - May also have nondisjunction in males (both males and females)
Explain Sex Linkage; for example, where does it occur?, how does it duffer from autosomal inheritance patterns?
- Example: eye colour in Drosophila is X-linked
- *there are also Y-linked inheritance patterns; however, many fewer due to less
genetic information on the Y chromosome - *sex-linked inheritance patterns contrast with autosomal inheritance patterns
*see different ratios of traits in different sexes *see differences in reciprocal crosses
X-linked Recessive Disorders: Red-Green Colour Blindness (3), What and How it occur?
- *light is perceived due to light-absorbing receptors in the eye
- *genes for the green and red light receptors are on the X chromosome (gene for the
the blue light receptor is autosomal) - *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)
Explain Human X-Linked Recessive Disorders. Frequency? Who affected? How Continued in the generations?
- Characteristics
. *relatively rare - *many more males than females show the phenotype
- *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
Example of X-Linked Recessive Disorders: Haemophilia A; what is it?
- abnormal or lack of blood clotting Factor VIII
- excessive bleeding, spontaneous bleeding in joints
Human Pedigree Analyses: X-linked Dominant Disorders - Characteristics (2), Example (Hypophosphatemia)
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
Human Pedigree Analyses: Y-linked
Characteristics (3)
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
Sex-Influenced Characteristics: WHAT OR HOW IS IT? inherited?
2 EXAMPLES: determined by? Recessive or Dominant in males/females?
- *controlled by autosomal genes
- *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
What are Sex-Limited Characteristics (3 and example) autosomal?
- *an extreme form of sex-influenced inheritance
- *controlled by autosomal genes
- *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
