genetics. chapter introduction how are traits passed from parents to offspring?
TRANSCRIPT
• Heredity– the passing of traits from parents to offspring.
Gregor Mendel
• Gregor Mendel is known as the father of genetics—the study of heredity.
Mendel studied genetics by doing controlled breeding experiments with pea plants… Why peas?
• Grow quickly
• Come in many varieties
• Mendel could control pollination
• There are two types of pollination:
Self pollination
Cross pollination
Mendel’s Experiments• Used plants that he knew were true-
breeding
• True breeding—when a plant self-pollinates it will always produce more plants with the same traits; (is homozygous)
(a.k.a. purebred)
X =
• Mendel crossed (mated) a true-breeding purple flowered pea plant with a true-breeding white flowered pea plant
• All of the offspring had purple flowers!
X =
(Parents) (Offspring)
What happened to the white flowers?
• (Transparency demo)
• Dominant trait—the trait that appears if two traits are mixed (capital letter)
• Recessive trait—trait that is hidden if 2 traits are mixed (lowercase)
• Purple flowers are dominant over white flowers
Mendel’s Experiments continued…
• Mendel also cross-pollinated hybrid plants.
X =
Parents
Offspring
75%
25%
Mendel’s Experiments continued…
• The trait for white flowers showed up again!
• He observed that offspring of hybrid crosses always showed traits in a 3:1 ratio.
75%
25%
Mendel’s Experiments continued…
Mendel concluded that two factors, one from each sperm and one from each egg, control each inherited trait.
• The factors that Mendel talked about are called genes
• A gene is a section on a chromosome that has genetic information for one trait.
Gene
Genes continued…
• One trait/gene may have a few different varieties
• Example: hair color
• The different forms of a gene are called alleles
brown
red
blonde
Genotype• The GENetic code (or combination of
alleles) of an organism is called its GENotype
• Shown as 2 letters per trait
• Example: someone with brown hair might be BB
One allele from mom
One allele from dad
• When the two alleles of a gene are the same, the genotype is true breeding, or homozygous.
• If the two alleles of a gene are different, the genotype is hybrid, or heterozygous.
What Controls Traits (cont.)
• The PHenotype is the PHysical appearance of the organism
• Example: brown hair, purple flowers, blue eyes
Phenotype
Check this out…
• B= brown hair b=blonde hair
• BB
• Bb
• bb
Brown Hair
Blonde Hair
Brown Hair
Homozygous (true-breeding)
Heterozygous (hybrid)
Homozygous (true-breeding)
Lets see how quick you are….• P = purple flowers in pea plants, p= white flowers in pea
plants
• PP
• Pp
• pp
Purple
Purple
White
What letters do you use?
• Choice of letter code is up to you…
• Usually the first letter of the trait
• Capital letter means the trait is dominant
• Lower case means it is recessive
A Punnett Square is a model used to predict possible genotypes and phenotypes of offspring.
•Bb x Bb
B
B
b
b
BB
Bb
Bb
bb
Probability- the mathematical chance an event will occur
• Bb x Bb
• Probability of Brown: 3 out of 4; 75%
• Probability of blue: 1 out of 4; 25%
B
B
b
b
BB
Bb
Bb
bbbrownbrown
brown blue
Phenotypic Ratio
•Bb x Bb
•Probability of Brown: 3 out of 4; 75%
•Probability of blue: 1 out of 4; 25%
•Phenotypic Ratio is 3:1 (Brown:blue)
B
B
b
b
BB
Bb
Bb
bb
brownbrown
brown blue
Genotypic RatioBb x Bb
•Probability of BB: 1 out of 4; 25%
•Probability of Bb: 2out of 4; 50%
•Probability of bb: 1 out of 4; 25%
•Genotypic Ratio is 1:2:1 (BB:Bb:bb)
B
Bbb bb
BB
B
Bb
b
brown
brown
brown
blue
A pedigree shows phenotypes of genetically related family members.
Modeling Inheritance (cont.)
• Sometimes traits appear to be blends of alleles.
• Alleles show incomplete dominance when the offspring’s phenotype is a blend of the parents’ phenotypes.
Complex Patterns of Inheritance
• Mixing of two traits (blending)
• One dominant allele is only “half-strong”
• Red + white = pink
R=red
r=white
• Codominance occurs when both alleles can be observed in a phenotype.
Complex Patterns of Inheritance
Codominance• Both alleles are expressed at the same
time in the phenotype
• Red fur + white fur = roan fur (red and white mixed)
R=red
r=white
