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Chapter 10: Mendel’s Laws of Heredity
What is Heredity?
Heredity
 The passing on of characteristics from parents to offspring through genes
Genes
 A section on DNA that carries the information on what type of protein to make;
they influence the types of traits an organism inherits
Traits
 Characteristics that are inherited
Genetics
 The branch of biology that studies heredity
History of Genetics
 Gregor Mendel: an Austrian monk that first developed the study of genetics
 Experimented with pea plants to discover the general laws of genetics
 A pea plant has gametes and both male and female reproductive organs
o Gametes: male and female sex cells
o Pollination: transfer of pollen (male gametes) from a male reproductive organ to a
female reproductive organ in a plant
 Mendel allowed pollination to occur within the same flower or between
different flowers for his experiments
 Fertilization: the male gamete unites with the female gamete to form a zygote (a
fertilized cell)
o In the case of pea plants, the zygote becomes a seed
 Conducted crosses between pea plants to compare traits
o Monohybrid crosses: when only one trait is compared at a time (like eye color)
o Dihybrid cross: when two traits are compared at a time (like eye & hair color)
P1 = Parental generation (parents)
F1 = First filial generation (children)
F2 = Second filial generation (grandchildren)
Mendel’s Conclusions
1. The rule of unit factors
* Each organism has two factors, or genes, that control each trait
o One gene comes from mom, one gene comes from dad
 A single gene can exist in different forms called alleles
o Example: Blue, brown, and green are alleles for the eye color gene
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2. The rule of dominance
* Offspring inherit an allele from each parent, but only one trait is observed
 Dominant trait: “stronger” trait that shows up when the dominant allele is present
o Represented by a capital letter; B is for brown eyes
 Recessive trait: “weaker” trait that shows up only when the dominant allele is not
present
o Represented by a lowercase letter; b is for blue eyes
The Law of Segregation
 What is means: Every organism has 2 alleles of each gene
 When gametes are made, each gamete receives only one of these alleles
 During fertilization, the offspring will receive one allele for each gene from each
parent
Ex. If the trait for height is represented by the allele “T” = tall and “t” = short,
Parents: Tt x Tt
Punnett Squares
 Shorthand way of determining the probability of having a certain type of offspring if
you know the parents’ genotypes
Phenotypes & Genotypes
 Phenotype: the physical appearance of an organism
o Ex. Brown eyes
 Genotype: genetic makeup of an organism
o Ex. Alleles for eye color are Bb or BB (= brown eyes)
 Homozygous: 2 alleles for a trait are the same
o BB is homozygous dominant for brown eyes
o bb is homozygous recessive for blue eyes
o Homozygous offspring are called purebred because their alleles are the same
 Heterozygous: 2 alleles for a trait are not the same
o Bb is heterozygous
o Heterozygous offspring are called hybrids because their alleles are mixed
Mendel’s law of independent assortment
 Genes for different traits are inherited independently of each other
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 Example: You have brown hair (Bb) and freckles (Ff)
o You can pass on to your children one of the following combinations: B + F,
b + F,
b+f
B + f,