Download Powerpoint

Inheritance: Mendel’s
Experiments
Ideas on Inheritance Before
Mendel
 Pangenesis
 Particles in the body are transmitted to the
reproductive cells
 Blending
 Black horse X White horse =Gray horse
 Homunculus (small human)
 Spermists believed a small fully formed
human was inside the sperm and the
female contributed nothing but her womb!
Gregor Mendel
(1822-1884)
 Augustinian monk
 Studied the inheritance of traits in over 20,000
pea plants
 Kept meticulous notes on his data
 No knowledge of DNA or chromosomes
 Father of genetics
 His genius was not recognized until after his death
The Lingo of a Geneticist
 Gene
 The unit of heredity (ex. Gene for flower color)
 Alleles
 Alternate forms of a gene (ex. Purple or white flower
color)
 Dominant vs. Recessive
 One allelic form “masks” the presence of another
 ex. Purple flowers are dominant over white flowers
(white is recessive)
 Dominant =capital letters
 Recessive =lower case letters
 Pn and Fn
 Parental and Filial generations
More Lingo
 Genotype vs. Phenotype
 Genotype is what alleles an individual has (ex. One purple flower
allele and one white flower allele)
 Phenotype is what the individual looks like (ex. Purple flowers)
 Homozygous vs. Heterozygous
 Homo =same
 Hetero =different
 True-breeding
 Phenotype of parent is always seen in offspring
 The phenotype counterpart to the genotype term “homozygous”
 If an individual has a homozygous genotype, it will be true breeding
 Monohybrid vs. Dihybrid
 A monohybrid cross looks at just one trait in parents who are
hybrids
 A dihybrid cross looks at more than one trait in parents who are
hybrids
 Hybrid is another way of saying “heterozygous”
Monofactorial Inheritance
 A.k.a. Mendelian traits
 Inheritance of a trait is determined by one
gene
 Does not really exist
 Virtually all traits are affected to some degree
by other genes and by environmental factors
Pea Plant Traits
Flower color
Purple (P)
White (p)
Mendel’s Experiments
(Pea Sex)
The Gene for Stem Length
 The tall allele is dominant (T)
 A true-breeding tall plant is homozygous
dominant (TT)
 The short allele is recessive (t)
 A true-breeding short plant is homozygous
recessive (tt)
Homozygous (True-breeding)
Dominant
Tall plant X Tall plant
X
All offspring will be tall
Homozygous (True-breeding)
Recessive
Short stem plant X Short stem plant
X
All offspring will be short
Non-True-Breeding
Tall plant X Tall plant
X
Some offspring are tall
Some offspring are short
Monohybrid Cross –Stem Length
Parental generation (P1)
Tall X Short
(Both true-breeding)
F1
All Tall
(Non-true-breeding)
a.k.a. hybrids
F2
¼ Short ¾ Tall
Truebreeding
Tall
Truebreeding
Short
Non-truebreeding Tall
Monohybrid Cross -Flower Color
Mendel’s Law of Segregation
 Elementen separate into gametes
 We now know “elementen” as genes
 Each individual carries 2 alleles for every gene
 One on each chromosome
 One allele on the chromosome from mom and one allele on the
chromosome from dad
 Possibilities for any given gene
 Homozygous Dominant (AA) (True breeding)
 Heterozygous (Aa) (Non-true-breeding)
 Homozygous Recessive (aa) (True-breeding)
 Gametes carry just ONE allele for each gene
 Possibilities are A or a
Our Old Friend Meiosis
Germline Cells
Phenotypic ratio
The genotypic ratio is 1:2:1
The Punnett Square
 Used to determine phenotypic and genotypic ratios
 Represents possible alleles in gametes and how
gametes may combine
Probability of having an
offspring with “tt” is the
same (1/4) for every
pregnancy/fertilization.
#s are ratios, not # of
offspring
Example
 True-breeding (homozygous) dominant
Dominant and Recessive Traits
 A dominant trait requires only one allele in
order for the dominant trait to be
expressed
 There MUST be 2 recessive alleles
Flower color
present in order for a recessive trait to be
expressed (phenotype)
Purple (P)
White (p)
Examples
 Ex. 1: True-breeding (homozygous) recessive
 Ex. 2: Homozygous dominant X heterozygous
 Ex. 3: Monohybrid (heterozygous) cross
Genotype vs. Phenotype
The Test Cross
X
Example
 Unknown X Homozygous Recessive
 a.k.a “the test cross”
Autosomes vs. Sex Chromosomes
 Autosomal traits are found on non-sex chromosomes so
(they are not found on the X or Y sex chromosomes)
Modes of Inheritance
 Autosomal dominant inheritance
 If a child exhibits the trait (phenotype) at least
one of the parents also exhibits the trait
(phenotype)
 Autosomal recessive inheritance
 Heterozygotes are carriers (genotype) and do
not exhibit the trait (phenotype)
Autosomal Dominant
Autosomal Recessive
Straight Hair as an Example
Autosomal Recessive
Phenylketonuria as an Example
Mendel’s Conclusions
 There are discrete units of inheritance
(elementen)

We now call these genes
 Offspring inherit a unit (gene) for each
trait from each parent
 Different units (genes) assemble
independently in the gametes