Chapter 4: Extensions and Modifications of Basic Principles Flashcards
What states does sexual reproduction alternate between?
Haploid and diploid states
What produces haploid gametes?
Meiosis
What produces a diploid zygote?
Fertilization (fusion of gametes)
How do male and female gametes differ?
In size and shape
How many pairs of chromosomes do humans have? How many autosomes?
- 23
- 22
What chromosome do males possess? What about females?
- Male: XY
- Female: XX
What is the SRY gene? Which chromosome is it located on?
- Y chromosome
- Causes the development of male characteristics
- Females do NOT possess the SRY gene
Which experimentation allowed us to discover that the SRY gene encoded for male characteristics?
- In mice, the SRY gene was knocked-down, which resulted in the loss of male characteristics in a male mouse
- When the SRY gene was inserted in a female, the mouse gains male characteristics
What allows the X and Y chromosome pairing in males?
- X and Y chromosomes are homologous at the pseudoautosomal regions
- Similarity is enough to allow pairing of X-Y chromosomes
What is the ratio of XX-XY sex determination?
1:1 sex ratio
What is the prevalence of Turner syndrome?
1 in 3000 females
What are the symptoms of Turner syndrome?
- Short
- Low hairline
- Broad chest
- Normal intelligence
- Most likely sterile
What is the genetic problem in Turner syndrome? What is the cause?
- Women only have a single X chromosome in their cells (XO)
- Due to problems at the cohesion level, the homologous chromosomes don’t dissociate, which creates unbalanced gametes.
What is the prevalence of Klinefelter syndrome?
1 in 1000 males
What are the symptoms of Klinefelter syndrome?
- Small testes
- Reduced facial and pubic hair
- Tall
- Sterile
- Normal intelligence
What is the genetic problem in Klinefelter syndrome?
- Men have a Y chromosome and two or more X chromosomes in their cells (XXY, XXXYY, XXXY)
- Due to non-disjunction as in Turner syndrome
What is the prevalence of Poly-X (Triplo-X) syndrome?
1 in 1000 females
What are the symptoms of Poly-X syndrome?
- No distinctive features
- Tall and thin
- Few are sterile (incidence is only slightly higher)
- Few are mentally challenged (incidence is only slightly higher)
- Often, women live their lives without knowing they have Poly-X
What is the genetic problem in Poly-X?
- Multiple X chromosomes (XXX, XXXX, XXXXX)
- The severity of intellectual disability increases as the number of X chromosomes increases beyond 3
What is androgen-insensitivity syndrome?
- When a person who is genetically male (one X and one Y chromosome) is resistant to male hormones (called androgens)
- The person has some or all the physical traits of a woman, but the genetic makeup of a man
What organism uses the XX-XO sex determination system?
Grasshoppers
In the XX-XO sex determination system, what is a male and what is a female? What might males also possess?
- Female: XX
- Male: XO (heterogametic) or X (hemigametic)
- There is no O chromosome - the letter O signifies the absence of a sex chromosome
What organism uses the ZZ-ZW sex determination system?
Birds, snakes, amphibians, fish
In the ZZ-ZW sex determination system, what is a male and what is a female? What might males also possess?
- Female: ZW (heterogametic)
- Male: ZZ (homogametic)
What is a genic sex-determining system? What is it found in?
- No sex chromosomes, only sex-determining genes
- Found in some plants, fungi, protozoans, and fish
Give an example of environmental sex determination in Crepidula fornicata.
- As the environment of the limpet changes, the sex will too
- Males are always at the top of the stack
Differentiate chromosomal, genic, and environmental sex determinations.
- Chromosomal: males and females have chromosomes that are distinguishable
- Genic: sex is determined by genes, but the chromosomes of males and females are indistinguishable
- Environmental: sex is determined by environmental factors
Give an example of genic sex determination in Drosophila melanogaster.
- X:A is the ratio of the number of X chromosomes over the number of haploid sets of autosomes
- Male: 0.5 ratio
- Female: 1 ratio
How many chromosomes does Drosophila melanogaster have?
- 3 pairs of autosomes
- X and Y
XY and XO produce what sex in Drosophila, if there is AA autosomes?
Male
XX and XXY produce what sex in Drosophila, if there is AA autosomes?
Female
XXX, XXXY and XXXX produce what sex in Drosophila, if there is AA autosomes?
Metafemale
XX produces what sex in Drosophila, if there is AAA autosomes?
Intersex
XO produces what sex in Drosophila, if there is AAA autosomes?
Metamale
Name some contributions to genetics by Drosophila melanogaster.
- Basic principles of heredity including sex-linked inheritance, multiple alleles, epistasis, gene mapping, etc.
- Mutation research
- Chromosome variation and behaviour
- Population and behavioural genetics
- Genetic control of pattern formation
What are X or Y-linked genes?
- A locus that is present on either the X or the Y, but not on the other
- Allows for differences in the segregation of the alleles
What causes the transformation of a “female” into a male during puberty (Guevedoces)?
- They have on the Y chromosome a rare allele of a gene that encodes for an enzyme that is involved in the synthesis of testosterone
- Since this enzyme is defective, the fetus will not have a first peak of testosterone
- There will only be a peak of testosterone at puberty, allowing them to grow male genitalia later in life
What is dosage compensation?
- Phenomenon that equalizes the amount of protein produced by X-linked genes in the two sexes
- The activity of the gene must be equalized in certain organisms
- Some organism won’t do anything though
Provide examples of dosage compensation.
- Halving the activity of genes (worms)
- Doubling the activity of genes on the X chromosome of males (fruit flies)
- Inactivation (placental mammals)
What is the genetic cause behind tortoise cats?
- Found in mostly females (rare for males to be affected)
- Patch of cells that carry one type of allele for the fur colour
- Another patch of cells carry a different allele, and produce a different colour
- Dosage compensation: inactivation of one X chromosome in a given cell (random = patches)
- Due to the Lyon hypothesis (inactivation of X chromosome)
What is the optimal proof that shows that white eyes in Drosophila are X-linked?
- The proof comes from a RECIPROCAL cross
- If a male has white-eyes (and female has red), it produces a 3:1/
- If a male has red-eyes (and female has white), a 1:1 ratio is produced
- In X-linked inheritance, the ratios are different depending on which sex is carrying the trait
What is a reciprocal cross?
A pair of crosses between a male of one strain and a female of another, and vice versa
What are the limitations to the study of genetics in humans?
- Controlled mating is not possible
- Long generation time
- Human family size is generally small
What do geneticists often use?
Pedigrees
What are the characteristics of a pedigree?
- Male: square
- Female: circle
- Affected: red
- Deceased: line crossed
What is an obligate carrier? What is the symbol?
- Carries the gene but does not have the trait
- Red circle in the center of the square/circle
What is a proband? What is the symbol?
- First documented affected family member coming to attention of geneticist
- P incoming into the shape
What is an asymptomatic carrier?
- Unaffected at this time but may later exhibit the trait
- Ex: in Huntington’s disease
What is Waardenburg syndrome characterized by?
- Deafness
- Fair hair
- Visual problems
- White forelock
How is Waardenburg syndrome inherited?
As an autosomal dominant trait
What are the characteristics of autosomal recessive diseases?
- Normally appear with equal frequency in both sexes
- Tend to skip generations
- More likely to appear among progeny of related parents
Cystic fibrosis, sickle cell anemia, and Tay Sachs disease are examples of what?
Autosomal recessive diseases
What are the characteristics of autosomal dominant traits?
- Appear in equal frequency in both sexes
- Do not skip generations
- Affected people have at least one affected parent
Huntington’s disease is an example of what?
Autosomal dominant disease
What is Huntington’s disease?
Brain disorder that causes uncontrolled movements, emotional problems, and loss of thinking ability
What are the characteristics of X-linked recessive traits?
- Appear more often in males than in females
- Are NOT passed from FATHER to SON
- An affected male can pass the allele to a daughter, who is unaffected, and passes it to sons who are
Hemophilia and Duchenne muscular dystrophy are examples of what?
X-linked recessive traits
What is hemophilia?
- Rare genetic bleeding disorder
- Caused by a shortage of certain clotting factors
What are the characteristics of X-linked dominant diseases?
- Affect both male and females
- An affect male must have an affected mother
- Do not skip generation
- Affected males pass the trait to all daughters, but none to sons
- Affected females (if heterozygous) pass the trait on to about half of their sons and daughters
What are the characteristics of Y-linked diseases?
- Only in males
- Passed from a father to all his sons
Rett syndrome is an example of what?
X-linked dominant disease
What is Rett syndrome?
- Neurological disorder that mainly affects females
- Caused by mutations in the MECP2 gene located on the X chromosome
What is retinitis pigmentosa an example of?
Y-linked disease
What is retinitis pigmentosa?
- Eye disease that causes severe vision impairment
- Mutated cells of the retina make defective proteins
- Gene is linked to the Y chromosome
Differentiate X-linked dominant and autosomal dominant pedigrees.
In X-linked dominant, whereas affected females can pass on the trait to either sons or daughters, affected males pass on the trait only to all daughters
What kind of trait is most likely to appear in a family with consanguinity?
Recessive traits
Define dominance.
Phenotype of the heterozygote is the same as the phenotype of one of the homozygotes
Define incomplete dominance.
Phenotype of the heterozygote is intermediate (falls within the range) between the phenotypes of the two homozygotes
Define codominance.
Phenotype of the heterozygote includes the phenotypes of both homozygotes
Define penetrance.
The proportion of individuals carrying a variant (or allele) of a gene (the genotype) that also express an associated trait (the phenotype)
Define expressivity.
In those who display the phenotype, it is the degree to which the phenotype is expressed (ex: a little lump of flesh vs. an entire limb)
What do multiple alleles at a locus allow for? What is necessary to observe these effects?
- For a greater variety of genotypes and phenotypes
- Observing a large group is necessary
What is the equation that defines the number of possible genotypes if there are multiple alleles at a locus?
[n(n + 1)]/2 where n is the number of different alleles
What is gene interaction?
Two different genes (two sets of alleles) contribute to the determination of a single characteristic (phenotype)
Provide an example of gene interaction.
- There are two genes encoding for the enzymes for the synthesis pathway of the pigments of peppers
- Each locus encodes an enzyme for a specific step in the exact same pathway, allowing them to affect the same characteristic
Does gene interaction affect the phenotypic ratio?
No
Define epistasis.
The effect of gene interaction in that one gene masks or hides the effect of another gene at a different locus
Explain recessive epistasis in a Labrador retriever.
- The E gene is responsible for taking the pigment from the shaft into the hair
- If the dog is homozygous for e, it is not capable of pushing the pigment out
- ee masks the effects of other genes
Define recessive epistasis.
The Recessivealleles at one locus (aa) mask the phenotypic expression of other gene locus
Define dominant epistasis.
The dominantallele of one gene masks the effects of either allele of the second gene
Explain dominant epistasis in squashes.
- When you have a dominant W allele, the enzyme is not functional
- The enzyme is responsible for producing the color of compound B
- The color remains compound A
What is duplicate recessive epistasis?
Two recessive alleles mask the genotype
What is the modified dihybrid phenotypic ratio in recessive epistasis?
- 9:3:4
- A_B_: 9
- A_bb: 3
- aa_B and aabb: 4
What is the modified dihybrid phenotypic ratio in dominant epistasis?
- 12:3:1
- A_B_ and A_bb: 12
- aaB_: 3
- aabb: 1
What is the modified dihybrid phenotypic ratio in duplicate recessive epistasis?
- 9:7
- A_B_: 9
- A_bb, aaB_, aabb: 7
Define sex-influenced characteristics.
- Determined by autosomal genes
- Inherited according to Mendel’s principles
- Expressed differently in males and females
Define sex-limited characteristics.
- Encoded by autosomal genes that are expressed in only in one sex
- The trait has zero penetrance in the other sex
What are the four characteristics of cytoplasmically inherited traits?
- Present in males and females
- Usually inherited from one parent, typically the maternal parent
- Reciprocal crosses give different results
- Exhibits extensive phenotypic variation, even within a single family
Which characteristics do not apply to Mendelian rules?
Cytoplasmically-inherited
Where are cytoplasmically-inherited characteristics encoded in?
Genes found on the mitochondrial or chloroplast genome
Why don’t cytoplasmically-inherited characteristics not follow Mendelian rules?
- No mitochondria in the sperm, some in the egg
- Unequal share of genetic information
- Information largely acquired from the mother
If mitochondrial genome is only acquired by the mother, then wouldn’t there be a large number of individuals with the same mitochondrial genome?
No, since the mitochondrial genome undergoes multiple generations, allowing it to be unique for each individual
Define the genetic maternal effect.
The phenotype of the offspring is determined by the genotype of the mother
When do genetic maternal effects arise?
When substances present in the cytoplasm of an egg (encoded by the mother’s nuclear genes)
Which generation determines the genotype in the genetic maternal effect?
The F1 generation mother determines the trait for the F2 generation
Define epigenetics.
The study of heritable changes in gene expression (active vs. inactive genes) that does not involve changes to the underlying DNA sequence
Define genomic imprinting.
Genes whose expression is affected by the sex of the transmitting parent
What affects the way DNA is expressed in genomic imprinting?
- Whether a gene passes through the egg or the sperm determines how much methylation of the DNA takes place
- The amount of methylation determines whether the gene is expressed in the offspring
Is the male gene or the female gene expressed in Igf2?
- Male gene is expressed
- Female gene is silent
Describe an example of how the expression of a genotype may be influenced by environmental effects?
- Certain genes are more solicited at a higher temperature
- For example, in the Himalayan rabbit, colder temperatures cause black fur at the extremities, and warmer temperatures cause white fur
How is sex predicted in a fruit fly? How is sex actually determined?
- Predicted by the X:A ratio
- Actually determined by genes on the X chromosome
What is a Barr body?
Inactivated X chromosome that appears as a condensed, darkly staining structure in most cells of female placental mammals
What is the Lyon hypothesis?
If a cell contains more than two X chromosomes, all but one of them is inactivated
How many Barr bodies will a male with XXXYY chromosomes have in each of his cells? What are those Barr bodies?
- Two Barr bodies
- Each Barr body is an inactive X chromosome
What are the causes of incomplete penetrance and variable expressivity?
The effects of other genes and environmental factors that can alter or completely suppress the effect of a particular gene
How does incomplete dominance differ from incomplete penetrance?
- Incomplete dominance: heterozygote is intermediate between the homozygotes
- Incomplete penetrance: some individuals do not express the expected phenotype
Define a lethal allele.
Allele that causes the death of an individual organism, often early in development, so that individuals do not appear in the progeny of a genetic cross
What are the effects of recessive and dominant lethal alleles?
- Recessive: kill individual organisms that are homozygous for the allele
- Dominant: kill both heterozygotes and homozygotes
What are the overall effects of lethal alleles?
Modify the ratios of progeny resulting from a cross
What is a compound heterozygote?
Possesses two different alleles that result in a recessive phenotype
A number of all-white cats are crossed, and they produce the following types of progeny: 12/16 all-white, 3/16 black, and 1/16 gray. What is the genotype of the black progeny? A) Aa B) AaBb C) A_B_ D) A_bb
D) A_bb
If the ratio is 9:3:4, what gene interaction occurred?
Recessive epistasis
If the ratio is 9:7, what gene interaction occurred?
Duplicate recessive epistasis
If the ratio is 12:3:1, what gene interaction occurred?
Dominant epistasis
What kind of characteristics have a higher penetrance in one sex over another?
Sex-influenced characteristics
In humans, what will be the phenotype of a person with XXXY sex chromosomes?
A) Klinefelter syndrome
B) Turner syndrome
C) Poly-X female
A) Klinefelter syndrome
How does incomplete dominance differ from incomplete penetrance?
A) Incomplete dominance refers to alleles at the same locus; incomplete penetrance refers to alleles at different loci
B) Incomplete dominance ranges from 0 to 50%; incomplete penetrance ranges from 51 to 99%
C) In incomplete dominance, the heterozygote is intermediate between the homozygotes; in incomplete penetrance, heterozygotes express phenotypes of both homozygotes
D) In incomplete dominance, the heterozygote is intermediate between the homozygotes; in incomplete penetrance, some individuals do not express the expected phenotype
D) In incomplete dominance, the heterozygote is intermediate between the homozygotes; in incomplete penetrance, some individuals do not express the expected phenotype
