Transcript Notes

Chapters 9 & 12:
Genetics
Heredity – The passing of traits from
parents to offspring
Genetics – The study of heredity
Gregor Mendel
• Austrian monk
• Bred pea plants
• 1860 - developed laws
of heredity
mms://204.13.204.36/Video9/mendelslaw.asf
•He cross-pollinated plants
•He bred plants to be pure for certain traits
•Ex: Tall parent  tall offspring
Short parent  short offspring
•Then he cross-bred plants with opposite
traits
•Tall x short
•Round x wrinkled
•Yellow x green
•Parents - P generation – tall x short
•Offspring – F1 generation (1st filial
generation)
•All offspring were tall (short trait
disappeared)
•Allowed F1 generation to self-pollinate
•F2 generation – 75% tall to 25% short
(short trait reappeared)
•Repeated many times – always same ratios
for each generation (see results slide #2)
Mendel’s Conclusions:
•There are 2 factors for every trait (today
we know these factors to be genes – 1
from mother, 1 from father)
•One of these factors can be dominant
over the other (the recessive trait)
•This is known as the Law of
Dominance
•The factors separate when the gametes
(eggs & sperm) are formed –The Law of
Segregation
•Each gamete only has 1 factor from each
pair (haploid)
•Fertilization gives each new individual 2
factors again (diploid)
•Mendel then crossed pure plants that
differed in 2 traits
•Ex: yellow, round peas crossed with
green, wrinkled peas
•F1 generation always showed dominant
traits
•F2 generation had the following results:
(see next slide)
F2: 9 yellow, round
3 yellow, wrinkled
3 green, round
1 green, wrinkled
•Based on these results, Mendel concluded that pairs
of factors separate independently during meiosis –
The Law of Independent Assortment
•Ex: Below, hairline and finger length are not
dependent on each other
• Alleles - various forms of a trait
• Ex: tall and short height
curly and straight hair
brown and blue eyes
Widow’s peak
The Epicanthal Fold (eye fold)
Genotype and Phenotype
• Genotype refers to the genes of an
individual; can be represented by two letters
• Homozygous - both alleles are the same
• Homozygous dominant - WW
• Homozygous recessive - ww
• Heterozygous – alleles are different - Ww
• Phenotype refers to the appearance of the
individual.
• Both WW and Ww result in widow’s
peak, the dominant trait
• ww will result in no widow’s peak, the
recessive trait
Monohybrid Crosses
• Considers only one trait.
• Punnett square – chart used to determine
probability
•Ratio shows #
of offspring
with dominant
vs. recessive
trait
Probability
• Determine the odds of an event occurring.
• Expressed as fraction or percentage
• Ex: (1/4) or 25%
• The probability that two or more independent
events will occur together is the product of
their chances occurring separately
• Ex: odds of having a boy = ½
Odds of having 2 boys = (1/2) x (1/2) =
(1/4)
• The chance of widow’s peak:
• WW or Ww = 75% or ¾
• Chance of a continuous hairline:
• ww = 25% or 1/4
•Odds of
having 3
children with a
continuous
hairline:
•(1/4) x
(1/4) x (1/4)
= (1/64)
Dihybrid Cross
• Two traits are considered
• Genotypes of the parents require four letters
(two for each trait).
• Codominance - both alleles are equally
expressed in a heterozygote
• Ex: Blood type – AB blood
• Incomplete dominance – heterozygous
genotype shows an intermediate
phenotype, representing a blending of
traits.
• Ex: Curly, wavy, or straight hair in
Caucasians
ABO Blood Types
• How your book shows blood type:
Blood type
(phenotype)
A
Genotype
IAIA or IA i
B
IBIB or IBi
AB
IA IB
O
ii
•How your teacher shows blood type:
Blood type
(phenotype)
A
Genotype
AA or AO
B
BB or BO
AB
AB
O
OO
Inheritance of blood type
Incomplete dominance
• Other examples of incomplete dominance:
• Plants called four o’clocks
RR – red
RR’ – pink
R’R’ – white
R
R
RR
• So a cross
RR’
R’
between two
pink plants produces
1 red, 2 pink, and 1 white plant
R’
RR’
R’R’
• Another example includes Sickle cell disease
in humans
• HbA represents normal hemoglobin; and HbS
represents the sickled condition
– HbAHbA – normal
– HbSHbS – sickle-cell disease
– HbAHbS - have the intermediate condition
called sickle-cell trait.
• Heterozygotes have an advantage in malariainfested Africa because the pathogen for
malaria cannot exist in their blood cells.
Sex determination:
•Female – XX
•Male – XY
•Always 50%
X
chance of having a
boy or a girl
Y
•Male determines
gender of baby
X
X
XX
XX
XY
XY
Sex-Linked Traits
• Traits controlled by genes on the X or Y
chromosomes
• X-linked or Y-linked
• Most X-linked traits are recessive, so a
female would have to have two recessive
genes to express the trait; a male would
only need one.
• Y-linked traits are only passed from
father to son
• Examples of X-linked traits include Color
blindness, Hemophilia, Muscular
Dystrophy, Fragile X Syndrome
Phenotype
Normal female
Carrier female
Affected
female
Normal male
Affected male
Gentoype
XBXB
XBXb
XbXb
XBY
XbY
Cross involving an X-linked allele
Pedigree Charts
• Constructed to show the pattern of
inheritance of a characteristic within a
family.
• The particular pattern indicates the
manner in which a characteristic is
inherited (suggests X-linked, dominant,
etc.)
Symbols used:
Normal female
Carrier female
Affected female
Normal male
Affected male
Male can be a carrier for an autosomal
trait.
Autosomal recessive pedigree chart
Autosomal dominant pedigree chart
Amniocentesis
• Uses a needle to extract amniotic fluid
from the uterus of a pregnant woman
from the 14th to 17th week of pregnancy.
• Up to 400 chromosome and biochemical
problems can be detected by culturing
fetal cells that are in the amniotic fluid.
• 0.3% chance of miscarriage with this
procedure – do this test only if certain
risk factors are present.
Amniocentesis
Chorionic Villi Sampling (CVS)
• Uses a thin suction tube to sample
chorionic cells from the placenta as early
as the fifth week of pregnancy.
• Chorionic cells are found in the
placenta
• The cells do not have to be cultured, and
karyotyping can be done immediately.
• 0.8% risk of miscarriage but can be
performed earlier than amniocentesis.
Chorionic villi sampling