Lecture 10: Meiosis

Question 1

meiosis

Answer 1

the basis of sexual reproduction
* depends on meiosis and fertilization
* produces offspring that contain a unique combination of genes from the parents

Question 2

different individuals of a single species have the same:

Answer 2

number and types of chromosomes

Question 3

human somatic cell

Answer 3

a typical body cell with 46 chromosomes

Question 4

karyotype

Answer 4

image that reveals an orderly arrangement of chromosomes

Question 5

homologous chromosomes

Answer 5

• matching pairs of chromosomes
• can have different versions of the same genes

Question 6

similarities and differences in humans and chimps

Answer 6

• both have 2 different sex chromosomes, X and Y
• humans have 22 pairs of autosomes (matching chromosomes)
• chimps have 23 pairs of autosomes

Question 7

autosomes

Answer 7

matching chromosomes

Question 8

what is the difference between homologous chromosomes and autosomes

Answer 8

in humans, the first 22 pairs of chromosomes are considered both autosomes and homologous chromosomes, with the 23rd pair being sex chromosomes which are not considered autosomes

Question 9

Oliver the Humanzee

Answer 9

• a former “performing” chimpanzee once promoted as a missing link or “humanzee” due to his somewhat human-like appearance and a tendency to walk upright
• purported to have 47 chromosomes (humans have 46 and chimps have 48), but this was false, as he had 48
• scientists found that Oliver was not a human-chimpanzee hybrid

Question 10

life cycle

Answer 10

in a multicellular organism, the life cycle is the sequence of stages leading from the adults of one generation to the adults of the next

Question 11

meiosis

Answer 11

• haploid daughter cells are produced in diploid organisms
• interphase is followed by two consecutive divisions, meiosis I and meiosis II
• crossing over occurs

Question 12

comparison of mitosis and meiosis

Answer 12

• in mitosis and meiosis, the chromosomes duplicate only once, during the preceding interphase

the number of cell divisions varies:
* mitosis uses 1 division and produces 2 diploid cells
* meiosis uses 2 divisions and produces 4 haploid cells

all the events unique to meiosis occur during meiosis I

Question 13

origins of sexual variation

Answer 13

offspring of sexual reproduction are genetically different from their parents and one another

Question 14

independent assortment of chromosomes

Answer 14

when aligned during metaphase I of meiosis, the side-by-side orientation of each homologous pair of chromosomes is a matter of chance

every chromosome pair orients independently of all of the others at metaphase I

for any species, the total number of chromosome combinations that can appear in the gametes due to independent assortment is 2^n, where n is the haploid number
* for a human, n = 23
* with n = 23, get 2^23
* there are 8,388,608 different chromosome combinations possible in a gamete

Question 15

crossing over

Answer 15

• nonsister chromatids of homologous chromosomes exchange corresponding segments
• genetic recombination (production of gene combinations different from those carried by parental chromosomes) occurs

Question 16

genetic recombination

Answer 16

the production of gene combinations different from those carried by parental chromosomes

Question 17

nondisjunction

Answer 17

• the members of a chromosome pair fail to separate at anaphase, producing gametes with an incorrect number of chromosomes
• nondisjunction can occur during meiosis I or II
• if nondisjunction occurs, and a normal sperm fertilizes an egg with an extra chromosome, the result is a zygote with a total of 2n + 1 chromosomes

Question 18

what happens if an organism with nondisjunction survives

Answer 18

if the organism survives, it will have
* an abnormal karyotype and
* probably a syndrome of disorders caused by the abnormal number of genes
* most miscarriages are due to abnormalities in chromosome number

Question 19

what happens when errors occur in meiosis

Answer 19

mistakes can result in genetic abnormalities that range from mild to fatal, such as nondisjunction (the alteration of chromosome number)

Question 20

down syndrome

Answer 20

• also called trisomy 21
• a condition in which an individual has an extra chromosome 21
• affects one out of every 700 children
• the incidence of Down syndrome in the offspring of normal parents increases markedly with the age of the mother

Question 21

nondisjunction in meiosis

Answer 21

• can lead to abnormal numbers of sex chromosomes
• seems to upset the genetic balance less than usual numbers of autosomes, perhaps because the Y chromosome is very small and carries relatively few genes