Chapter 2 Flashcards
Mendel’s Experiments
- Ensured phenotypes were all distinct and contrasting, and all controlled by single-gene inheritance
- All plants were pure line (all offspring produced by matings within the members of that line were identical)
Mendel’s Model
- A gene is necessary for producing pea colour
- Each plant has a pair of this type of gene
- Gene comes in 2 forms (alleles)
- A plant can either be YY, Yy, or yy
- Phenotype of Yy is always yellow, Y is dominant over y
- Members of a gene pair separate equally into the cells that become sperm and eggs in meiosis: Law of Equal Segregation
- At fertilization, gametes fuse randomly, regardless of which alleles are present
What is a zygote?
Fertilized egg
What is a homozygote?
plant with a pair of identical alleles for a gene i.e AA or aa
What is a heterozygote?
- plant with different alleles for a gene, Aa
- sometimes called monohybrid
What is homozygous dominant?
AA
What is heterozygous?
Aa
What is homozygous recessive?
aa
What is a genotype?
allele combinations underlying phenotypes
What is a monohybrid cross?
Aa x Aa
What are meiocytes?
Specialization cells that divide to produce sex cells i.e eggs and sperm
What are null alleles?
alleles that make proteins with zero function
What are leaky mutations?
Reduced level of enzyme function -> some wild type functions “leak” into mutant phenotype
What are silent mutations?
no functional impact, basically wild type
What does haplosufficient mean?
1 gene copy has enough function to produce a wild-type phenotype
What does haploinsufficient mean?
Null mutant allele will be dominant because single wild-type allele alone can’t provide enough product for normal function
What does new function mean?
When mutation results in a new function that is dominant in a heterozygote because the wild type cannot mask the new function
Null mutations are recessive in:
halposufficient genes, and dominant in halpoinsufficient
Depending on their action, mutant alleles can be:
dominant or recessive, question of dominance needs to be considered in analysis.
A dominant mutation in the heterozygous state will:
be expressed
A cross between heterozygous dominant and wild-type will result in:
1:1 phenotypic ratio
Principles of inheritance can be applied in 2 ways:
- Inferring genotypes from phenotypic ratios
2. Predicting phenotypic ratios from parents of known genotypes
What is the homogametic sex?
Females, have a pair of sex chromosomes (XX)
What is the heterogametic sex?
Males, have a non-identical pair of sex chromosomes (XY)
Inheritance patterns of genes on sex chromosomes are different than:
those of autosomal genes
What is a dioecious species?
- A species that shows animal-like dimorphism, females bearing flowers have only ovaries and male bearing flowers have only anthers.
- Some have non-identical sex chromosomes, some have no visibly different sex chromosomes
Cytogeneticists divide X and Y chromosomes into:
differential and homologous regions
Differential regions contain:
the most genes and have no counterparts on the other sex chromosome
Genes in the differential region are:
hemizygous (half-zygous), most genes don’t take part in sexual function
Y chromosome only contains:
A few dozen genes, most don’t have counterparts on the X chromosome
What is sex linkage?
- non-autosomes-sex chromosomes
- one sex does not have a pair of similar sex chromosomes, the other does
What is X linkage?
When mutant alleles in the differential region of X chromosome show a single-gene inheritance pattern
What is Y linkage?
When mutant alleles of few genes in differential region of Y chromosome have linkage
A gene that is sex-linked can show:
Phenotypic ratios that are different in each sex
X and Y chromosomes in human have 2 short:
homologous regions, one on each end
What are pseudoautosomal regions 1 and 2?
Homologous regions that are autosomal-like, one or both of these regions pairs with the other sex chromosome during meiosis and undergoes crossing over
Pseudoautosomal regions 1 and 2 allow:
X and Y chromosomes to act as a pair and separate into equal numbers of sperm
Males only need to inherit a single:
X-linked recessive allele to express it, whereas females need 2
How is sex-linked inheritance recognized?
By different phenotypic ratios in the 2 sexes of progeny and different ratios in reciprocal crosses
What are autosomal recessive disorders?
When the affected phenotype is inherited as a recessive allele
What are the patterns for autosomal recessive disorders?
- Disorder appears in progeny of unaffected parents
2. Affected progeny includes males and females
What is the appearance of the pedigree for autosomal recessive disorders?
- Look bare with a few black symbols
- Affected siblings with ppl in earlier and later generations not affected
What are autosomal dominant disorders?
When the normal allele is recessive and the defective allele is dominant
What is the pattern for autosomal dominant disorders?
Phenotype appears in every generation of the pedigree, affected mothers and fathers pass onto sons and daughters
Why is the pattern for autosomal dominant disorders like that?
Appears in every generation because the abnormal allele carried by a person must have come from a parent in the preceding generation
What are polymorphisms?
The coexistence of 2 or more reasonably common phenotypes of a biological property
Morphs are often inherited as:
alleles of a single autosomal gene
What is the X-linked recessive disorder pattern?
- More males than females show rare phenotype
- None of the offspring of an affected male show the phenotype, but daughters are “carriers” -> next gen = 1/2 of males are affected
- None of the sons of affected fathers show phenotype and do not pass it on
What is the X-linked dominant disorder pattern?
- Affected males pass on condition to all females, but no sons
- Affected heterozygous females mating with unaffected male results in condition to 1/2 males and females
What is Y-linked inheritance?
- Only males inherit, from fathers passing it on
- Maleness is linked to the Y chromosome
- Inheritance patterns with an unequal representation of phenotypes in males and females can locate the genes connected to one of the sex chromosomes
What is a test cross?
- Cross used to determine the genotype of an individual that is expressing a dominant phenotype
- Crossed with tester which is homozygous recessive
Meiosis 1
Prophase 1: chromosomes condense
Metaphase 1: align on midline
Anaphase 1: homologous dyads pulled apart
Telophase 1: dyads arrive at pole and nucleus forms
Meiosis 2
Prophase 2: chromosomes condense
Metaphase 2: aligns on midline
Anaphase 2: sister chromatids pulled apart
Telophase 2: nuclei re-form, each chromatid now considered chromosome
