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
•He cross-pollinated plantsmms://204.13.204.36/Video9/mendelslaw.asf
•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, round1 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)
Genotype
A IAIA or IA i
B IBIB or IBi
AB IAIB
O ii
Blood type (phenotype)
Genotype
A AA or AO
B BB or BO
AB AB
O OO
•How your teacher shows blood type:
Inheritance of blood type
Incomplete dominance
• Other examples of incomplete dominance:
• Plants called four o’clocks
RR – red
RR’ – pink
R’R’ – white
• So a cross
between two
pink plants produces
1 red, 2 pink, and 1 white plant
RR RR’
RR’ R’R’
R R’
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 malaria-infested
Africa because the pathogen for malaria cannot exist in their blood cells.
Sex determination:•Female – XX•Male – XY•Always 50% chance of having a boy or a girl•Male determines gender of baby
XX XX
XY XY
X X
X
Y
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 Gentoype
Normal female XBXB
Carrier female XBXb
Affected female
XbXb
Normal male XBY
Affected male 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.)
Normal female
Carrier female
Affected female
Normal male
Affected male
Symbols used:
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