Download Mendel and Meiosis

Mendel
and
Meiosis
Learning Goal:
predict possible outcomes of various
genetic combinations such as monohybrid
crosses
Gregor Mendel



Heredity is the passing of characteristics
from parents to offspring.
Genetics is the study of heredity or how
those traits are inherited.
Gregor Mendel was the first person to
succeed in predicting how traits were
inherited thus he became known as the
Father of Genetics.
Gregor Mendel



Gregor Mendel is best known for his
study of pea plants.
He used pea plants because they have
2 distinct sex cells called gametes.
Mendel pollinated the flowers himself by
transferring the male pollen to the
stigma of the flower (pollination).
Pollen
Uniting of the male and
female gametes is
called fertilization.

Mendel cross-pollinated pea plants in
order to study the various traits:
Dominant:
the trait that
was
observed
Recessive: the
trait that
disappeared.
Genotypes and Phenotypes

Phenotype – the way that the organism
looks and behaves.
Pheno (physical) type (model)

Genotypes – the combination of alleles
(genes) in the organism.
Gen (birth) type (model)
Genotypes and Phenotypes


Allele – the different forms of a gene
ex: Blue eyes or Brown eyes
Alleles can be dominant or recessive
 Dominant



The trait covers up (masks) the recessive trait
Symbol is a capital letter (T)
Recessive


The trait is masked by the dominant allele
Symbol is a lowercase letter (t)
Genotypes and Phenotypes

Phenotype

Genotype
Alleles
Genotypes and Phenotypes

Homozygous dominant:
Homo (same)

Homozygous recessive:

Heterozygous:
Hetero (different)
Crossing traits




Hybrid: a cross between plants with different
traits like tall and short.
Mendel’s first cross he called a monohybrid
cross. (Mono means one)
Later Mendel began crossing plants with 2
traits. These crosses were called dihybrid
crosses.
Testcross: Breeding an individual of unknown
genotype to a homozygous recessive individual
to reveal the unknown genotype
Parent
generation
Filial
(son or daughter)
generation
2nd Filial
generation
Parents
Children
Grandchildren
Mendel’s Laws of Inheritance

Law of segregation: the 2 alleles for
each trait (on homologous
chromosomes)will segregate (separate)
with the formation of the gametes
during meiosis.
Mendel’s Laws of Inheritance


Law of independent assortment: Each
trait (or gene) is inherited independently
of the other traits.
In other words, all blondes do not have
blue eyes.
Punnett Squares


In 1905, Reginald Punnett, an English
biologist, devised a shorthand way of
predicting the outcomes of the various
crosses.
These are called Punnett
squares.
Monohybrid crosses
gametes
b
B
B
Bb
Bb
bb
b
B: White
b: Black
BB
Bb
Genotypes
Phenotypes
Bb: 100% Bb
100% White
Bb
Monohybrid crosses
B: White
b: Black
gametes
B
b
B
b
BB
Bb
Genotypes
Phenotypes
Bb
bb
BB: 25%
Bb: 50%
bb: 25%
White: 75%
(BB and Bb)
black: 25%
Genotypic ratio
1:2:1
Phenotypic ratio
3:1
Monohybrid crosses
BB: White
bb: Black
Bb
gametes
B
b
b
Bb
bb
b
Bb
bb
Genotypes
Phenotypes
BB: 0%
Bb: 50%
bb: 50%
50% White
50% Black
Genotypic ratio
0:1:1
Phenotypic ratio
1:1