Heredity Flashcards
particulate inheritance
(Mendel’s theory of genetics)
inherited characteristics are carried by discrete units (genes)
basics of probability
predicts average outcome if sample is large enough
law of dominance
(Mendel’s first law)
when 2 organisms, each homozygous (pure) for 2 opposing traits are crossed:
- offspring will be hybrid (carry 2 different alleles)
- only exhibits the dominant trait (hidden trait is the recessive trait)
law of segregation
during formation of gametes, 2 traits carried by each parent seperate
monohybrid cross
- (Tt x Tt) is cross between 2 organisms that are each hybrid for 1 trait
- phenotype (appearance) ratio from cross is 3 to 1
- genotype (type of genes) ratio is 1:2:1 (25% homozygous dominant, 50% heterozygous, 25% homozygous recessive)
backcross or testcross
way to determinte genotype of individiual showing only dominant trait
- involves cross between real organisms
- individual in question (B/?) crossed with homozygous recessive individual (b/b)
- homozygous dominant if all offspring is B/b
- hybrid if half offspring can show recessive trait
law of independent assortment
- cross carried out between 2 individuals that are hybrid for 2+ traits that are not on the same chromosome (dihybrid cross)
- during gamete formation, alleles of a gene for 1 trait will segregate independently from alleles of a gene for another trait
genes will not assort independently if genes are linked
the dihybrid cross
cross between individuals that are hybrid for 2 different traits
- cross produces 4 types of gametes: TY, Ty, tY, ty
- 16 genotypes, but a few phenotypes (ratio of 9:3:3:1)
incomplete dominance
- characterized by blending
- neither trait is dominant so the genes are written in all capital letters
ex: long watermelon (LL) crossed with round watermelon (RR) produces oval watermelonsd (RL)
when 2 RL are crossed, 25% chance for round, 25% chance for long, and 50% for oval
codominance
both traits show
ex: 3 blood groups: M, N, MN that are based on 2 distinctmolecules located on surface of red blood cells
- person can be homozygous for 1 type of moleucle (MM/NN), or be hybrid and have both molecules (MN)
multiple alleles
when there are more than 2 allelic forms of a gene
many genes in population exist in only 2 allelic forms
pleiotropy
ability of single gene to affect organism in many ways
epistasis
2 seperate genes control 1 trait, but 1 gene masks expression of other gene
gene that masks expression of other gene is epistatic to the gene
polygenic inheritance
characteristics that result from blending of several seperate genes
ex: skin color, hair color, height…
wide variation in genotypes always results in bell-shaped curve in population
genomic imprinting
variation in phenotype depending on if trait is from mother or father
- caused by silencing of particular allele by methylation of DNA during gamete formation
- imprint is carried to all body cells + passed down generation by generation
- imprinted genes located on autosomes instead of X chromosome (sex-linked genes)
extranuclear genes
genes located in mitochondria and chloroplasts, where DNA is small, circular, and carries small number of genes
- rare + severe diseases linked to extranuclear genes, defects in mitochondrial genes cause weakness in muscles
mitochondrial DNA inherited only from mother, father’s mitochondria does not enter egg
genes and environment
environment can alter expression of genes
penetrance
proportion/percentage of individuals in gruop with given genotype that shows expected phenotype
ex: ppl with mutant allele for BRCA1 develop breast cancer, but some people with the allele do not develop breast cancer
reason is unknown, but it might be interaction with some other gene/factor from environment
linked genes
genes on same chromosome
linked genes tend to be inherited together and do not assort independently (unless separated by crossover event during meiosis/gamete formation)
more genes than chromosomes, 1000s genes linked
humans have 46 chromosomes, so they have 46 linkage groups
sex-linkage
46 human chromosomes: 44 (22 pairs) autosomes and 2 sex chromosomes (X and Y)
- traits on X chromosomes sex-linked
- few genes carried on Y chromosome
- females (XX) inherit 2 copies of sex-linked genes
- if sex-linked trait is due to recessive mutation: female will express phenotype if she carries 2 mutated genes (X-X-) and will be a carrier if she only has one (X-X), will express phenotype with 1 mutated genes if mutation is dominant
- males (XY) will express phenotype if the one X is mutated (X-Y), and recessive traits (color blindness, hemophilia, Duchenne muscular dystrophy) are more common, so they are more likely to suffer with sex-linked conditions more often than females
- all daughters of affected fathers are carriers
- sons cannot inherit sex-linked trait from father because son inherits Y chromosome from father
- son has 50% chance of inheriting sex-linked trait from a carrier mother
- uncommon for female to have recessive sex-linked condition, needs to inherit mutant gene from both parentsr
crossover
- genes more likely to be seperated if they are farther apart
- see chiasma (physical bridge built around point of exchange)
- results in recombination
linkage mapping
1 map unit = distance within which recombination occurs 1% of the time
recombination frequencies
(number of recombinants/total number of offspring) x 100
pedigree
- family tree that indicates phenotype of 1 trait for every member
- used to determine how trait is inherited
- females = circle, males = square
- carrier state normally not shown, half-shaded if it is
- shaded shape = exhibits trait
X inactivation - The Barr Body
- in every somatic cell with more than 2 X chromosome, a random X chromsome will be inactivated
- inactive choromsome condenses into dark spot of chromatin and is seen in outer edge of nucleus (Barr Body)
- genetic mosaic: some cells have X inactivated, some cells have other X inactivaed
- calico cats are female bc of this
mutations
- any changes in the genome/genetic or chromosomal abnormality
- occurs randomly, even if radition and chemicals can cause them
- 2 types: gene mutations or chromosome mutations
gene mutations
change in DNA sequence
chromosome mutation
deletion - when fragment lacking a centromere is lost during cell division
inversion - when chromosomal fragment reattaches to original chromosome but in reverse orientation
translocation - when fragment of chromosome becomes attached to nonhomologous chromosome
polyploidy - when cell or organism has extra sets of chromosomes
can be seen under a microsope
- karyotype shows size, number, shape of choromosomes + reveals presence of abnormalities
- karyotype used to scan for chromosomal abnormalities in developing fetuses
nondisjunction
- error that sometimes occurs during meiosis, when homlogous chromosomes fail to seperate as they should
- can result in abnormal number of chromosomes (aneuploidy)
- chromosome present in triplicate (trisomy)
- organism wih cells with extra set of chromosomes is triploid (3n)
- organism with 4n chromosome number is tertraploid
- strawberries are octoploid
- organism with extra sets of chromosomes is polypoid
