Chapter 14 Flashcards
what is heredity?
passing on of traits from one generation to the next
what is the blending hypothesis?
genetic material contributed by parents mixes like paint
(yellow + blue = green)
why is the blending hypothesis not how heredity works?
- offspring traits not always average of parents
- some traits are skipped and return across generations
what is the particulate hypothesis?
parents pass on distinct “heritable factors”/genes to their offspring
Why are peas a good organism for testing out the particulate hypothesis?
- easy to grow
- have easily distinguishable phenotypes
- could strictly control mating between organisms to ensure parentage of offspring
what are “either-or” traits?
no intermediates, either one distinct character or another distinct one
ex: no green, either yellow or blue
what is the concept of true breeding?
breeding of offspring where the physical appearance of a parental generation is identical to the offspring generation
ex: purple parents, purple offspring
what are characteristics for good models when it comes to studying genetics/heredity?
GROS
G- generation time is rapid
R - reproduction is quick
O - offspring are plentiful
S - space taken up is small
what is a monohybrid cross?
cross between two individuals differing in A SINGLE character
what four hypotheses did Mendel develop to explain monohybrid cross conclusions? (3:1 in F₂ generation)
- alleles account for variations in inherited characters
- for each gene, an organism inherits two alleles, one from each parent
- if two alleles differ, the dominant allele determines organism’s phenotype
- law of segregation - two alleles for a heritable character separate during gamete formation and end up in different gametes
what are alleles?
alternate versions of genes
why do dominant alleles not suppress a recessive allele?
alleles don’t interact, they’re simply variations in a gene’s nucleotide sequence
what is the law of segregation?
two alleles for a heritable character separate during gamete formation and end up in different gametes, applies only to genes located on different chromosomes, or very far apart on the same chromosome.
Why are dominant alleles not necessarily more common than recessive alleles in the gene pool?
dominant alleles do not suppressive recessive alleles
an allele is dominant because it is seen in the phenotype, not because it can overcome recessive alleles
the segregation of alleles corresponds to what process?
chromosome separation in anaphase I during meiosis I
why doesn’t an organism’s traits always reveal its genetic composition?
phenotype/traits are not equal to genotype/genetic composition
(a dominant trait-expressing individual can be heterozygous or homozyogus dominant)
what is a testcross? what does it accomplish?
breeding a dominant expressing individual with unknown genotype (AA? Aa?) with a homozygous recessive individual (aa), determines genotype of an individual with a dominant phenotype
if offspring displays recessive phenotype (aa), unknown individual has heterozygous allele (Aa)
what are Mendel’s two laws of inheritance?
- law of segregation - following a monohybrid cross
- law of independent assortment - following two traits at the same time
what is the law of independent assortment?
two copies of each gene segregates into gametes independently of the two copies of another gene, genes do not influence each other with regard to the sorting of alleles into gametes: every possible combination of alleles for every gene is equally likely to occur
what are dihybrids?
crossing two true-breeding parents differing in two characteristics
what is a dihybrid cross?
cross between F₁ dihybrids
(AaBb x AaBb)
what does a dihybrid cross accomplish?
determines whether two characteristics are transmitted to offspring together as a package or independent of each other
what phenotypic ratio did Mendel observe that supports the hypothesis of independent assortment?
9:3:3:1
why does the law of independent assortment work as long as two genes are on two separate chromosomes?
if genes were on the same chromosome, both will be present on a gamete and therefore will not be independent of each other
the physical basis of the law of independent assortment lie in which process?
metaphase of meiosis I
equal probability of maternal and paternal chromosome facing a given pole
how does Mendel’s law of segregation and independent assortment reflect the rules of probability?
each event is independent, the outcome of one event has no effect on the outcome of the next event
the alleles of one gene segregate into gametes independently of another gene’s alleles
what is the multiplication rule?
“and” rule; states that the probability that two or more independent events will occur together is the product of their individual probabilities
what is the addition rule?
“or” rule; states that the probability that any one of two or more exclusive events will occur is the sum of their individual probabilities
How does the inheritance of characters by a single gene deviate from Mendelian patterns?
- alleles can be incompletely dominant or recessive
- a gene can have more than two alleles
3.a gene can produce multiple phenotypes
what are the degrees of dominance?
the degree to which an allele is expressed on the organism
complete, incomplete, codominance
what is complete dominance?
when the phenotypes of heterozygote and dominant homozygote are identical
what is incomplete dominance?
the phenotype of a hybrid is somewhere between the phenotypes of the two parental varieties
(white + red = pink offspring)
what is codominance?
the two dominant alleles affect the phenotype of an offspring in separate, distinct ways, both alleles are clearly present
(white + red = white and red offspring)
why are humans not good subjects for genetic research?
- very long generation time
- parents produce relatively few offspring
- breeding experiments are unacceptable
how can patterns of inheritance be inferred in humans?
analyzing family pedigrees
what is a pedigree?
a family tree that describes the interrelationships of parents and children across generations
what are characteristics of dominantly inherited traits on pedigrees?
- males and females equally affected
- affected offspring have atleast one affected parent
- affected offspring are HETEROZYGOUS if only ONE PARENT is affected
- unaffected offspring are homozygous recessive
- if one parent is heterozygous, about half of offspring will be affected
- trait DOES NOT SKIP generations
- affected parents can have unaffected child
why are dominant inherited alleles that cause lethal diseases rare?
the mutant dominant allele is likely to die out in a population because they usually result in the death of an organism before they have a chance to transmit the allele to its offspring
what are characteristics of recessively inherited traits on pedigrees?
- males and females equally likely to be affected
- affected offspring often have unaffected parents, which are heterozygous
- heterozygous individuals are CARRIERS
- if both parents heterozygous, a quarter of the offspring will be affected
- trait OFTEN SKIPS generations
are the sex chromosomes in humans fully homologous?
NO, the tips of the arms are the only regions of the sex chromosomes that are homologous
why are the sex chromosomes not fully homologous?
- different in size, X chromosome has more genes than Y chromosome
- almost none of the genes in the X chromosome have counterparts in the Y chromosome
what are X-linked genes?
genes present in the X-chromosome only
what is the probability of getting a male or female with random fertilization?
ALWAYS 50%, law of segregation explains how gametes in females will result in X-bearing eggs whereas gametes in males will result in a 1:1 ratio of X-bearing and Y-bearing sperm
is the sex of an offspring determined by sperm or egg in humans?
sperm, a gamete may bear Y-chromosomes containing info for male development whereas eggs will always bear X-chromosomes
why are fruit flies a convenient organism for genetic studies?
- breed at high rate
- short generation time
- have only four pairs of chromosomes
- polytene chromosomes
- males are XY and XX, similar to humans
what are wild type phenotypes?
phenotypes that are the most common in a population
genotype is denoted by a + sign
what are mutant type phenotypes?
phenotypes that are alternative to the wild type, different from the most common characteristic
genotype denoted by a - sign
why are X-linked recessive alleles most commonly expressed only in males?
most genes in the X chromosome have no counterparts in the Y chromosome. so a recessive mutation in an X-linked gene is expressed in males (b/c its the only gene present that can code for protein, it is dominant in a way)
what is crisscross inheritance?
an X chromosome with an X-linked gene can crisscross or alternate between the sexes in successive generations
- initially, male carried mutant gene
- after, female carries mutant gene
- male carries mutant gene in next generation
what is nondisjunction?
when chromosomes do not separate properly during Anaphase I or II
how does nondisjunction provide evidence that genes reside on chromosomes?
when X chromosomes in females occasionally fail to separate during anaphase I or II, eggs with abnormalities resulted in “exceptional” offspring that unexpected traits
why are chromosomal abnormalities deadly?
When there is a missing, extra, or irregular portion of chromosomal DNA, it can cause errors in the expression of these genes, leading to physical and mental abnormalities
what is trisomy?
3 copies of a chromosome in an affected individual
what are characteristics of X-linked recessively inherited traits on pedigrees?
- males are affected more frequently than females
- trait is NEVER passed down from FATHER to SON
- affected sons are usually born to CARRIER mothers
- about half the sons of a carrier mother will be affected
- All daughters of an affected male and an unaffected non-carrier female are 100% CARRIERS
- Trait OFTEN SKIPS generations
what are characteristics of X-linked dominantly inherited traits on pedigrees?
- males and females equally affected
- all daughters of an affected father are affected, but not the sons
- affected sons always have affected mothers
- about half the offspring of an affected other will be affected
- affected daughters can have affected mother or father
- trait DOES NOT SKIP generations
why are X chromosomes inactivated in female mammals?
because they have two X chromosomes, one of their X chromosomes will be inactivated randomly in each somatic cell during embryonic development
what is a barr body?
an inactivated and condensed X chromosome
why are barr bodies produced in female mammals?
female mammals have 2 X-chromosomes, 1 is condensed to avoid making twice the number of proteins needed
How are barr bodies condensed?
DNA Methylation
describe X-inactivation
- Xist gene transcription is increased
- Xist RNA from the transcription binds with the X-chromosome inactivation center (XIC)
- eventually coats entire chromosome
- coated chromosome triggers DNA methylation
How does DNA methylation slow/stop transcription?
when nucleotide bases are methylated (a methyl group is added to them), they restrict access to the genes, therefore inhibiting transcription
Why does the Y-chromosome determine the organism’s sex as male?
the sex determining region Y (SRY) gene on Y-chromosome encodes the protein SRY which triggers male development
What are characteristics of Y-linked inheritance on pedigrees?
- ONLY MALES exhibit trait
- females DO NOT inherit/transmit trait
- All sons of affected males will show the trait
what are haplotypes?
a group of alleles located on the same chromosome in an organism that are inherited together from a single parent
How can the ancestry of a male’s Y chromosome be traced?
By determining his Y-chromosome haplotypes
why does an organism’s mitochondria originate from the mother?
mother’s oocyte has cytoplasmic contents, including mitochondria. the father’s sperm only delivers his genetic info, very small cytoplasmic contents
What are the characteristics of a mitochondrial trait on pedigrees?
- Both males and females exhibit trait
- Males DO NOT transmit trait
- All the affected mother’s offspring are affected
What are linked genes?
genes that segregate together during meiosis, same arm, same chromosome
does independent assortment apply to linked genes?
No, linked genes are on the same arm and same chromosome
what are nonrecombinant groups?
parental groups; offspring that have the same genotypes as their parents.
what are recombinant groups?
offspring that have different genotypes compared to their parents as a result of crossing over/synapsis during prophase I
how does crossing over result in recombinant groups?
allele combinations that were originally together in each chromosome have recombined in the chromosomes that participated in the crossover
what does crossing over result in?
2 nonrecombinant chromosomes
2 recombinant chromosomes
why does crossing over only result in 2 recombinant strands and not 4?
crossing over only occurs between only two of the four strands
how can the frequency of recombination be used to map the relative position of genes on chromosomes?
the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency
how do you calculate the frequency of recombination?
(# of recombinant offspring) / (total # of offspring) x 100%
why is the max frequency of recombination 50%?
the random assortment of genes generates 50% recombination when the genes are so far apart that crossing over always takes place. half the offspring will have same genotypes as parents and half will have different genotypes. Any higher than 50% means that the genes are not linked at all
