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 ( )  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  P n and F n  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 White (p) Purple (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 (P 1 ) Tall X Short (Both true-breeding) F1F1 All Tall (Non-true-breeding) a.k.a. hybrids F2F2 ¼ Short ¾ Tall True- breeding Tall True- breeding Short Non-true- breeding 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 Flower color White (p) Purple (P)  There MUST be 2 recessive alleles present in order for a recessive trait to be expressed (phenotype)

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