Chapter 15 - Inheritance Flashcards
what are genes?
segments of DNA located along chromosomes
different genes are on different chromosomes
how are chromosomes and genes present?
chromosomes and genes are both present in pairs in diploid cells
homologous chromosomes separate and alleles segregate during the process of meiosis - after meiosis, fertilization restores the paired condition for both chromosomes and genes
what is the law of independent assortment?
alleles of genes on non-homologous chromosomes assort independently during gamete formation
the random arrangement of chromosome pairs at metaphase I accounts for independent assortment of the alleles for two or more genes located on different homolog pairs
what is the law of segregation?
the two alleles for each gene separate during gamete formation
separation of homologs during anaphase I accounts for the segregation of the two alleles of a gene into separate gametes
what is the chromosome theory of inheritance?
according to this theory, Mendelian genes have specific loci (positions) along chromosomes and it’s the chromosomes that undergo segregation and independent assortment
where does Mendelian inheritance have its physical basis
in the behavior of chromosomes
which organism did Morgan use to prove that chromosomes are indeed the location of Mendel’s heritable factors?
fruit fly: drosophila melangaster
advantages of using the fruit fly
- a single mating will provide hundreds of offspring
- it only has four pairs of chromosomes: 3 pairs of autosomes and 1 pair of sex chromosomes
what is the wild type?
the phenotype for a character most commonly observed in natural populations, such as red eyes in Drosophila
what are mutant phenotypes?
traits that are alternatives to the wild type, such as white eyes in Drosophila
this is because they’re due to alleles assumed to have originated as changes or mutations in the wild-type allele
what identifies a wild type genotype?
for a given character in flies, the gene takes its symbol from the first mutant discovered –> the allele for white eyes in Drosophila is symbolized by w
a superscript (+) identifies the allele for the wild-type trait
what was Morgan’s experiment? what were the results?
1) he mates a white eyed male fly with a red eyes female
2) all the F1 offspring had red eyes (wild-type allele is dominant)
3) when he bred the F1 flies to each other, he got the classical 3:1 phenotypic ratio among F2 generation BUT there’s a plot twist, the white-eye trait showed up only in males - all the F2 females had red eyes while half the males had red, and half had white
conclusion: fly’s eye color is liked to its sex –> furthermore, since only males in the F2 generation had white eyes, Morgan deduced that this eye color gene is located on the X chromosome and that there is no corresponding locus on the Y
what did Morgan’s experiment support?
**provided support for the chromosome theory of inheritance: namely, that a specific gene is carried on a specific chromosome
also indicated that genes located on a sex chromosome exhibit unique inheritance patterns
what are the different sex systems?
mammals: XY
grasshoppers, cockroaches, insects: females XX, males X
birds, fish insects: females ZW, males ZZ
halpo-diploid: no sex chromosomes in most species of bees/ants –> females develop from fertilized eggs are are diploid, males develop from unfertilized eggs and are haploid
which chromosome is bigger in a human?
the Y chromosomes is much smaller than an X
short segments on either end of the Y chromosome are the only regions that are homologous with regions of the X which allow the X and Y to pair and behave like homologs during meiosis –> The X and Y chromosomes share small regions of DNA homology that allow them to pair (synapse) and undergo crossing over at meiosis in males
what is SRY
a gene for sex-determining region of Y
in the absence of SRY, the gonads develop into ovaries
Y directs the male pathway –> The SRY gene on Y directs testis development –> Testis produce sex hormones that determine maleness
what is a sex-linked gene?
a gene located on either sex chromosome; those located on the Y chromosome are called Y-liked genes
Y-linked genes
Y chromosome is passed along virtually intact from a father to all his sons
because there are so few Y-liked genes, very few disorders are transferred from father to son on the Y chromosome
what are X-liked genes
the human X chromosome contains approximately 1,100 genes which are called X-liked genes
fathers pass X-liked alleles to all of their daughters but to none of their sons
inheritance of X-liked genes
if an X-liked trait is due to a recessive allele, a female will express the phenotype only if she is homozygous for that allele (XnXn)(if XNXn then she’ll only be a carrier)
any male receiving the recessive allele from his mother will express the trait because his genotype will be XnY (males have more X-liked recessive disorders than females)
color blindness is an X linked gene
what is Duchenne muscular dystrophy?
if affects about 1/3500 males in the US
it’s characterized by a progressive weakening of the muscles and loss of coordination
the disorder is traced to the absence of a key muscle protein called dystrophin and have mapped the gene for this protein to a specific locus on the X chromosome
what is hemophilia?
an X-liked recessive disorder defined by the absence of one or more of the proteins required for blood clotting - when a person with hemophilia is injured, bleeding is prolonged because a firm clot is slow to form
what are examples of X-linked disorders?
- color blindness
- duchenne muscular dystrophy
- hemophilia
X inactivation in female mammals
- almost all of one X chromosome in each cell in female mammals becomes inactivated during embryonic development = males and females have the same dose of most X-liked genes