Download Genetics - TeacherWeb

Genetics
AP Biology
2002
Vocabulary
-Allele: alternate form of a gene; symbolized by a letter
-Gene: sequence of nucleotides that code for a certain trait
-Phenotype: outward expression of a gene
-Genotype: the alleles an organism has
-Homozygous: 2 of the same alleles; either dominant or recessive
-Heterozygous: 2 different alleles
Inheritance patterns
-Complete dominance: one allele can completely mask another; not always the most
common phenotype
-Dominant allele can hide another; is symbolized by a capital letter which is chosen for
the dominant phenotype
-Recessive allele is hidden by the dominant; is symbolized by a lower case letter
-TT x TT, TT x Tt, TT x tt, Tt x Tt, Tt x tt, tt x tt
Incomplete dominance
-Heterozygote shows a blending of the parental traits
-CRCW , RR’, RW
Co-dominance
-Heterozygote shows both parental traits
-MN blood type: MM has only M proteins, NN has only N proteins, MN has both M and
N proteins; Roan cows, etc
Multiple alleles
-More than 2 alleles for a given trait’
-ABO blood type
-Pheno.
Geno.
Antigen
-A
IAIA or IAi
A
B
B
-B
I or I i
B
-AB
IAIB
A, B
-O
ii
none
antibodies
B
A
none
A,B
Mendel
-Was a monk in Austria in mid-1800’s, studied math and science
-As he was tending peas in the garden, he began to notice patterns in the pea
characteristics ( tall or short, white or purple flowers, etc)
-Did a series of experiments over a 10 year period of time—mating, growing and
counting pea plants with differing characteristics (30,000 pea plants total) and compared
them to the previous and next generations
-Concluded that some “factor” was passed between generations that could be hidden but
not lost
Mendel’s Laws
-Law of segregation: homologous chromosomes separate and are packaged into different
gametes
-Law of independent assortment: each pair of homologous chromosomes lines up
independently of each other during meiosis 1
Probability rules
-Multiplication rule: the probability of independent events are a product of their
independent probabilities
-Addition rule: to determine the probability of an event that can occur 2 or more different
ways you add the individual probabilities
Monohybrid crosses
-One trait on one chromosome
-Pp x Pp; ½ the gametes are P, ½ are p
-pp = ½ x ½ = ¼
-PP = ½ x ½ = ¼
-Pp can be done 2 ways: P x p or p x P
-¼ + ¼ = ½
Dihybrid crosses
-2 traits on 2 separate chromosomes; treat as independent events
-Parental cross: PPtt x ppTT
-F1: all offspring will be PpTt; mate the F1’s
-PpTt x PpTt (same as Pp x Pp and Tt x Tt)
What are the phenotype ratios?
-Purple, tall P__T__
-Purple short P__tt
-White tall
ppT_
-White short pptt
Trihybrid crosses and beyond
-AaBbCcDd x AaBbCcDd
-What is the probability of producing offspring that is heterozygous for all the traits?
-Desired outcome: AaBbCcDd
-Aa x Aa = ½ Aa, Bb x Bb = ½ Bb, Cc x Cc = ½Cc, Dd x Dd = ½ Dd
-½ x ½ x ½ x ½ = 1/16
-What is the probability of producing a homozygous recessive offspring?
-¼ x ¼ x ¼ x ¼ = 1/256
Epistasis
-A gene at one locus alters the phenotypic expression of another gene (stands on it)
-Ex: In mice the gene for pigment deposition C is epistatic to the gene for pigment
production
-CC or Cc—melanin can be deposited; cc—albino
-BB or Bb—black; bb—brown
-Genes are on separate chromosomes and assort independently
Polygenic inheritance
-Traits are determined by many loci so there is a range of phenotypes
-ex: skin pigmentation in humans is controlled by 3 genes—A, B, C—which show
incomplete dominance; the more capital letters, the darker the skin color (the more
melanin is produced)
Sex Chromosomes
-Humans have 23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex
chromosomes
-Females are XX, males are XY (X and Y look different from each other); females give
all their eggs one X chromosome plus cytoplasm and organelles;
Sex determination
-Males determine the sex of the child since ½ of their sperm get the X and ½ get the Y
-Any information on the X chromosome will appear in males, whether recessive or
dominant; females require 2 recessive alleles to show a recessive trait
-Information solely on the Y chromosome are called holandric genes (porcupine quill
body hair, hairy ear rims, SRY gene); only effect men
Barr Bodies
-In body cells of females one X chromosome at random is turned off early in
development; inactivated X is called a Barr body
-All the cells descended from that cell have the same X turned off
-If female is heterozygous she becomes a mosaic—some areas have the dominant gene
expressed, some have the recessive
-Ex: calico cats, patches of colorblindness
Non-disjunction
-Failure of chromosome pairs to separate during meiosis
-Results in gametes with too many or too few chromosomes
-Aneuploidy: abnormal # of a certain chromosome
-Polyploidy: more than 2 complete chromosome sets
-An embryo needs at least one X chromosome to survive
Sex linkage problems
-It is now important if the offspring is male or female so the X and Y have to be used
along with superscripts to show the alleles
-Examples of X-linked disorders: colorblindness, hemophilia, muscular dystrophy, eye
color in fruit flies
Hemophilia
-H-normal, h-hemophiliac
-Phenotype
-Normal female
-Carrier female
-Hemophiliac female
-Normal male
-Hemophiliac male
Genotype
XHXH
XHXh
XhXh
XHY
XhY
Crossing over
-The degree of crossing over of any 2 loci is proportional to the distance between them.
-In complete linkage only the parental type gametes are produced
-In gametes produced by crossing over, two new combinations appear (the result of nonsister chromatids exchanging segments) along with the parental types
-The closer the loci are to each other the less recombinants are formed.
Example
-Yellow, white, and miniature mutations in fruit flies
-Data--crossing over observed:
-Yellow, white = .5%
-White, miniature = 34.5%
-Yellow, miniature = 35.4%
-What is the order of the genes?
-Yellow and white must be relatively close because of the low recombination frequency.
-Miniature must be far apart from both yellow and white because the recombination
frequency is high.
-Because frequency of miniature and yellow is greater that miniature and white, white
must be between them
-Order is yellow—white—miniature