Unit 5 - Chromosomal Theory of Inheritance

theory of inheritance

genes are located on chromosomes, chromosomes segregate and independently assort during meiosis

evidence by cytologists

1879 mitosis worked out, 1890 meiosis worked out

evidence by geneticists

1860 proposed laws of segregation and independent assortment, 1900 Mendel’s work re-discovered

what do sex-linked genes exhibit

unique patterns of inheritance

how is sex determined

by presence of Y chromosome in humans

XX

female

XY

male

in sex linkage some traits why are only some traits inherited

they are on the X chromosome

what type of traits are sex linkage typically

recessive

do males or females inherit sex linkage more often

males

sex linked disorders

colorblindness, hemophilia, Duchenne Muscular Dystrophy

sample cross colorblindness

X^C is normal vision, X^c is color blind, X^CX^c times X^cY, in any sex linked cross affected males Xc get from mom because males get X chromosome from mom, affected females get one X^C from mom and one from dad, males affected more than females

why do linked genes tend to be inherited together

they are located near each other on the same chromosome

where are linked genes located

on the same chromosome

what do linked genes not do

assort independently

when do chromosomes cross over

during gamete formation

what do the number of crossovers observed indicate

able to determine the order of genes and relative distance of genes from one another

what does crossing over result in the offspring

offspring may or may not look like their parents, parental types do, recombiant types don’t

un-linking genes

crossing over can un-link genes on the same chromosome depending on distance from each other

what can cause genetic disorders

alterations of chromosome number or structure

nondisjunction

whole chromosome mutation, failure of chromosomes to separate during meiosis

disorders due to nondisjunction

monosomy, trisomy

monosomy

turner syndrome, female who is XO

trisomy

down syndrome (trisomy 21), klinefelter syndrome is males who are XXY

changing poloidy can change —

phenotype, affects all pairs

what is ploidy common in

plants ex: increased vigor in polyploids

what can changing ploidy result in

higher yield of fruit, greater resistance to disease, greater resistance to drought, ex: triploid (3n) sterility, seedless watermelon, seedless grapes, seedless bananas

chromosomal alterations

mutations of parts of chromosomes, replication error or crossing over error, deletion, duplication, inversion, translocation

some inheritance patterns are exceptions to standard —

Mendelian inheritance

not all genes are found on —

nuclear chromosomes

what organelles have circular DNA molecules that carry genes

mitochondria and chloroplasts

why is the circular DNA almost always passed on from the mother

the egg houses theses organelles while the sperm only carries chromosomes