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Unit 4 Exam
Chapter 10: Meiosis
Inheritance
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Genes​=hereditary units, segments of DNA
o Locus​=a gene’s specific location along the length of a chromosome
Gametes​=the vehicles for transmission of genes to offspring
o Sperms and eggs…come together in fertilization
o This means they have to have only ½ the genetic complement-1/2 from mom and ½ from
dad
Somatic cells​=all “body” cells other than gametes-46 chromosomes
o Gametes will only have 23 chromosomes

Studying chromosomes
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Karyotype​-ordered display of chromosomes arranged in pairs; picture taken during metaphase
when DNA is duplicated and condensed
Homologous chromosomes​-paired based upon having same length, same centromere position,
same staining (banding) pattern
Autosomes​-pairs 1-22
Sex chromosomes​-pair 23, XX or XY
Diploid​- 2n, double set of chromosomes
Haploid​- n, gametes, one set of chromosomes
Sister chromatids​-totally identical, duplicates (both together look like one side of the x-shaped
pair); completely connected along entire length of chromosome

Life Cycles
● Life cycle​-generation to generation sequence of stages in an organism’s reproductive history
● Fertilization and meiosis alternate in a sexual life cycle
● Three major types of sexual life cycles:
o Animals/human
▪ Germ cells (and only germ cells) generate haploid gametes (sperm or eggs) via
meiosis
▪ Fertilization of an egg develops into a diploid zygote
▪ Mitosis leads to a diploid multicellular organism
o Plants/Algae
▪ Alternation of Generations
▪ Has both diploid (sporophyte) and haploid (gametophyte) multicellular stages
● Alternates between them
● Almost like two separate plants
o Fungi/Protists
▪ Only diploid stage is a single-celled zygote

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No multicellular diploid

Meiosis
● Cutting the number of chromosomes from diploid to haploid
● Two division stages-meiosis I and meiosis II
o Both similar to mitosis, especially meiosis II
● Prophase I:
o Synapsis​ occurs when the paired homologous chromosomes are physically connected
along their lengths by a zipper-like complex called the synaptonemal complex
▪ Genetic recombination​ (​crossing over​) occurs during synapsis
● Exchange of corresponding segments of DNA from non-sister chromatids
● Chiasma​-the x-shaped region where crossing over has occurred (plural is
chiasmata)
● Metaphase I:
o Chromosomes again line up at midline, but two-by-two in homologous pairs
● Anaphase I:
o Each homologous chromosome goes its own way
o Split homologous pairs, not sister chromatids
● Telophase I and Cytokinesis:
o Thought to occur simultaneously
o At this point, we have two daughter cells that ARE haploid, but DNA is just duplicated
▪ DNA is still duplicated: two (not finished) identical (haploid) copies
● Meiosis II is just like mitosis x2, but we finish with ​four haploid cells that are NOT genetically
identical​ (due to crossing over)
Comparing Meiosis and Mitosis
● Three major events unique to mitosis: (all during meiosis I)
o Synapsis and crossing over
o Homologous pairs at the metaphase plate
o Separation of homologues (Anaphase I), rather than sister chromatids
Genetic Variation
● Two goals of meiosis:
o Cut chromosomes in half
o Produce genetic variation
● Meiosis produces genetic variation
● Mutations​ naturally occur, serving as a source of genetic diversity that gives rise to different
versions of genes known as ​alleles
o 9/10 are not detrimental; natural changes in DNA
● Reshuffling of alleles during sexual reproduction produces the variation that results in each
member of a population (makes you different form your sibling)
● Sexual reproduction uses three major mechanisms to contribute to genetic variation
o Independent Assortment of Chromosomes

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o

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When homologous pairs line up at the metaphase plate, they consist of one
maternal and one paternal chromosome
▪ Each pair will line up with paternal oriented toward one pole and maternal
oriented to the other, but each pair will do this independently of the other pairs
▪ At the end of meiosis I, the two daughter cells will be a random mixture of DNA
that originated with the mother and father of the individual whose cells are
undergoing meiosis
▪ If a cell has n=2, then the possible combinations are 2^n=2^2+4
● Two possible arrangements for each of 2 chromosome pairs
▪ So, in human cells where there are 23 pairs (n=23), there are 2^23=84 million
possibilities
Genetic recombination
▪ Happens first chronologically
▪ So independent assortment gives us a collection of gametes that are new
combinations of our parents’ chromosomes
▪ In addition to this, crossing over generates ​recombinant chromosomes​, individual
chromosomes that carry genes from two different parents
Random fertilization
▪ Random joining of a particular egg and sperm adds a final layer of complexity
▪ Each gamete is one of 8
...
99% of the population is homozygous recessive
▪ Normal height people are dd
o Huntington’s Disease
▪ Case of a lethal dominant allele that can pass on because it causes death at an
advanced age
▪ Degenerative disorder of the nervous system with no phenotypic symptoms until
middle age
▪ 1 in 10,000 people
X-linked disorders
o Generally recessive, so seen almost exclusively in men
▪ Female require two “broken” X’s, while men only need one
o Colorblindness
o Duchenne muscular dystrophy
▪ 1 in 3500 males in US
▪ Lack of muscle protein dystrophin leads to progressive weakening of muscles, loss
of coordination, and death in 20’s
o Hemophilia
▪ Absence of protein(s) from the clotting cascade
▪ “Royal Disease”
Chromosomal Abnormalities​ can be the result of:
o Abnormal chromosome number

Nondisjunction​-homologous chromosomes do not separate during meiosis I, or
sister chromatids do not separate during meiosis II
▪ Resulting zygote is said to have ​aneuploidy
● May be ​monosomic​ if derived from a zygote lacking a chromosome (2n-1)
● May be ​trisomic​ if derived from a zygote with an extra chromosome
(2n+1)
o Example is down syndrome (trisomy 21)
▪ 1 in 700 children born in US
▪ Characteristic facial features, short statures, heart defects,
developmental delays; increased risks of Alzheimer’s and
leukemia
▪ With treatment, live fairly normal life span
▪ Risk increases with age of mother
● 0
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92% over 40
▪ Other trisomies rarely survive even to birth (trisomy of 13
and 18 may be live born, but lethal shortly thereafter)
● Aneuploidy of Sex Chromosomes
o XXY=Klinefelter syndrome-1 in 500-1000
▪ Still male, female characteristics (less body hair, wider
hips, more breast development), low intelligence
o XYY=just taller-1 in 1000
o XXX=Trisomy X-also just taller-1 in 1000 women
o Monosomy X=Turner Syndrome (X0)-1 in 2500
▪ Only know viable monosomy in humans
▪ Sterile, potentially lower intelligence
▪ Polyploidy​ occurs if a cell has more than two complete sets
● Triploidy (3n) and tetraploidy (4n) are common in plants (but lethal for
animals)
Alterations of chromosome structure
▪ Not losing/gaining entire chromosome
▪ Types:
● Deletion: removes a chromosomal segment
o Cri du chat (“Cry of the Cat”)
▪ Chromosome 5
▪ Brain/head malformation, quickly lethal (some in throat,
scream sounds like cat)
● Duplication: repeats a segment
● Inversion: reverses a segment within a chromosome; does not matter as
much
● Translocation: moves a segment from one chromosome to a
non-homologous chromosome
o Implicated in many cancers
o Chronic Myelogenous Leukemia (CML)
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o

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Translocation in mitosis of white blood cell
predecessors…chromosomes 9 and 22
Produces distinctive shortened 22, called Philadelphia
chromosome

Differences between meiosis and mitosis



AaBbCC-write different possible combinations



Need to know what no disjunction means, etc



Know example of each type of disease (autosomal recessive, X linked, trisomy, etc)



Know human life cycle-know plants is called alternation of generations


mammal: multicellular diploid, haploid gamete


plants: alternation of generations-multicellular diploid and haploid


fungal: multicellular haploid, diploid spore (cell); almost inverse of our cycle



Epistasis:


gene with either/or expression


Bug: Gg wing color G=green g=yellow


totally different chromosome=Ww for wings W=wings w=no wings


so if you have no wings, the color doesn't Matter



Know multi factorial: genetic sand the environment play a part


hair after light exposure



Polygenic: quantitative characters-number of genes that help determine height, and then big range of heights



Norm of reactions: phenotypes aren't just cut and dry, one thing, environment can affect it



Be ready to look at data and notice the ratios that you are looking for


Bears can be brown or black



3000 male black



2500 female black (5500 black)



500 male brown



610 female brown (1110 brown)


dominant or recessive?-black is dominant; about the 3:1 ratio


autosomal or sex linked?- not sex linked


how could you test to see if right?-go Mendel, try to find pure black and pure brown, mate hybrid offspring


if 1000 brown male and 110 brown female-could say X linked



Be ready to look at results and not just make a Punnett square



No grid in



Questions may be worded a little differently



Keep in head autosomal or sex-linked? Dominant or recessive?


may not directly ask it, but say what does this pedigree show you about this trait?




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