Mendelian Genetics Unit 1
Mendelian
Genetics
Unit 1
Genetics
Science that deals with the structure & function of genes and their transmission from one generation to the next
(heredity)
Genes
factors that control traits
Genotype
Genetic makeup of an organism
Phenotype
Observable characteristic of an organism produced by the interaction between its genotype and the environment
Genotype & Phenotype
Genes provide only the POTENTIAL for developing a particular phenotype.
The extent to which it is realized depends upon
Interactions with other genes & their products
Environmental influences
Random developmental events
Genes are only a starting point for determining structure and function of an organism.
Gregor Mendel
Father of modern genetics
Experimented using garden pea plants (Pisum
sativum)
Good choice:
Easy to grow
Bears flowers and fruit in the same year a seed is planted
Produces a large number of seeds
Each trait had two easily distinguishable, alternative appearances
Mendel’s Work
Pea plants are normally self-fertilizing
He allowed each strain to self-fertilize for many generations to ensure that the traits he wanted to study were inherited (truebreeding strains).
Then he prevented self-fertilization and cross fertilized true-breeding strains of peas that differed in a single trait
This is a MONOHYBRID CROSS
Used reciprocal crosses to show that the trait does not depend on the sex of the organism
Example:
Cross smooth female with wrinkled male
Cross wrinkled female with smooth male
Cross of the P (parental) generation:
All the F
1 progeny exactly resemble only one of the parents (not a blend of both)
Mendel planted and allowed the
F
1
plants to self-fertilize:
Both smooth and wrinkled seeds appeared in the F
2 generation (3:1 ratio).
But how can a trait present in the P generation disappear in the F
1 generation and then reappear in the F
2
?
Mendel concluded…
The alternative traits in the cross were determined by what we now call genes .
Factors transmitted from parents to progeny that carry hereditary information
Each existed in alternative forms (which we now call alleles )
A true-breeding strain of peas must contain a pair of identical factors (and each F1 must have contained both factors).
Because only one of the traits was seen in F1, the expression of the missing trait must somehow have been masked by the visible trait: DOMINANCE
The allele for purple (P) is dominant to the allele for white flowers (p).
The allele for white flowers is recessive because it is masked.
Individuals that contain two copies of the same specific allele of a particular gene are said to be HOMOZYGOUS for that gene.
Individuals that have two different alleles of a particular gene are said to be
HETEROZYGOUS.
Punnett Square
Matrix that describes all the possible genotypes of progeny resulting from a genetic cross
Mendel’s Conclusions:
Results of all reciprocal crosses were the same.
All F
1 progeny resembled one of the parental strains
(indicating dominance).
In the F
2 generation, the parental trait that had disappeared in the F
1 generation reappeared.
The two members of a gene pair (alleles) segregate (separate) from each other during the formation of gametes.
• We now know genes are on chromosomes and the specific location of a gene on a chromosome is called its locus .
• Gene segregation parallels the separation of homologous pairs of chromosomes at anaphase I in meiosis.
Confirming the Principle of Segregation
Test Cross
Cross of an individual of unknown genotype (usually expressing the dominant phenotype) with a homozygous recessive individual to determine the unknown phenotype
