AP Biology Chapter 15 Study Guide

Law of Segregation

Homologous Chromosomes split during Anaphase I

-2 alleles for each gene separate

Law of Independent Assortment

Nonhomologous chromosomes line up independently during Metaphase I

Thomas Hunt Morgan

-Studied Fruit Flies (Drosophila melanogaster)

-Produce many offspring

-Generation can be bred every two weeks

-Only have 4 pairs of chromosomes (2n=8, n=4)

-Took 2 years to obtain white eyed flies

Wild Type

Normal/Common Phenotypes

-W+=Red eyes

Mutant Type

Alternative phenotypes

-W=White eyes

Male

XY

Female

XX

Morgan's Experiment

P generation: X^W Y x X^W+ X^W+ Male white eyes X Female red eyes

F1 generation: All red eyes

F2 generation: 3:1 red to white eye ratio, only males had white eyes

X Chromosomes

Have 1,100 genes

• X-linked: hemophelia, red-green color blindness, muscular dystrophy

Y Chromosomes

78 genes

• Y-linked: SRY (Sex-determining region on Y-chromosome) responsible for teste development in embyro, determines if the offspring is a male

-X & Y chromosomes are nonhomologous

Chromosome Systems

-Humans: XY System (XX Female, XY Male)

-Chickens: ZW System (ZW Female, ZZ Male)

-Grasshopper: XX System (XX Female, X Male)

-Bees: Haplo-diploid system (32 Diploid Fertilized Female, 16 Haploid Non fertilized Male)

Sex-Linked Gene

A gene that is located on either sex chromosome

-X chromosomes are x-linked

-Y chromosomes are y-linked

X Inactivation

In females, one of the 2 X chromosomes in each cell is randomly inactivated during embyronic development

Example: Sweat glands and mosaic patterns

Barr body

A condensed X chromosome

Linked Genes

Genes located on the same chromosome

Genetic Recombination

Production of offspring with combinations of traits differing from either parent

Parental Types

Offspring with a phenotype matching one of the parental phenotypes

Recombinant Types

Offspring with nonparental phenotypes

-Due to crossing over in Prophase I of Meiosis I

Recombination Frequency

-50% frequency for 2 genes on different chromosomes

-Used for gene mapping

of recombinants/total # of offspring * 100 = Recombination frequency

Crossing over

Crossing over between homologous chromosomes during Meiosis I leads to recombinants

Genetic Map

Ordered list of loci along a particular chromosome

Sturtevant's Prediction

The farther apart 2 genes are, the higher the probability a crossover will occur between them and therefore the higher the recombination frequency

-More distance -> More crossing over in a larger area -> more recombinants -> higher recombination frequency

-Proved that genes are found on chromosomes

Linkage Map

A genetic map of a chromosome based on recombination frequencies

Map Units

Distance between genes

1% recombination frequency = 1 map unit

-Indicate RELATIVE distance and order, not precise location of genes

Chromosomal Alteration

-Lead to miscarriages in humans

-Plants can tolerate it

Nondisjunction

-Pairs of homologous chromosomes do not separate normally during meiosis

-Anaphase I: Homologous chromosomes don't separate, 100% bad gametes

-Anaphase II: Sister chromatids don't separate, 50% bad gametes (Preferable)

-One gamete receives 2 of the same chromosome, and another gamete receives no copy

-Not common

Aneuploidy

Results from the fertilization of gametes in which nondisjunction occurred

-Extra Individual Chromosomes

-Offspring with this condition have an abnormal number of a particular chromosome

-Bad sperm or egg from nondisjunction + Good egg or sperm

Monosomy (Aneuploidy)

-Monosomic zygote has only one copy of a particular chromosome (n-1)

-Turner Syndrome (X0) : Only viable monosomic disease in humans, monosomy of sex chromosome X

Trisomy (Aneuploidy)

-Trisomic Zygote has 3 copies of a particular chromosome (n+1)

-Trisomy 21/Down Syndrome: Aneuploid condition that results in 3 copies of chromosome 21

a) Frequency increases with age of mother, has not been explained

b) Described by facial features, short stature, fixable heart defects, sterile sex organs, and more

Polyploidy

-Condition in which an organism has more than 2 complete SETS of chromosomes

-Triploidy (3n): 3 sets of chromosomes

-Tetraploidy (4n): 4 sets of chromosomes

-Common in plants: Bananas (3n), Wheat (6n), Strawberries (8n) and some fish

-More normal in appearance than aneuploidies

Alterations of Chromosome Structure

-Deletion: Removes a chromosomal segment (Cri du chat)

-Duplication: Repeats a segment, occurs in S-phase

-Inversion: Reverses orientation of a segment within a chromosome

-Translocation: Moves a segment from one chromosome to another, occurs in non-homologous chromosomes (Chronic myelogenous leukemia (CML), chromosomes 9 & 22)

Aneuploidy of Sex Chromosomes

-Nondisjunction of sex chromosomes doesn't bother humans that much

-XXX: Females are healthy, at risk for learning disabilities

-XXY: Kleinfelter syndrome, result of extra X in male, Testes are smaller, they are taller and lean

-X (X0): Turner Syndrome, produces sterile Females

-XYY: Males with no bad symptoms, taller

Cri Du Chat

Specific deletion in chromosome 5

-Severe intellectual disability and a catlike cry

-Usually die in infancy or early childhood