Genetics Nov 17
Genetics
Learning Objectives
Know basic genetic terminology.
Predict genotypic and phenotypic ratios involving monohybrid and dihybrid crosses.
Properly and efficiently use Punnett squares and probability rules.
Understand how meiosis explains Mendel’s Laws of Segregation and Independent Assortment.
Gregor Mendel – The “Father” of Genetics
Studied the inheritance (transmission) of physical traits across generations in the pea plant.
Observed that traits were inherited (transmitted from one generation to the next) in predictable patterns.
Concluded that hereditary information passes from one generation to the next in discrete units.
Discrete units identified by Mendel are called genes.
Examples of Traits Studied by Mendel
Yellow seed color
Green seed color
Wrinkled seed shape
Round seed shape
Purple flower color
White flower color
Terms Used in Genetics
Genes: Heritable units of information that determine or influence observable or measurable traits.
Examples:
Gene ‘A’
Gene ‘B’
BRCA1 gene
CFTR gene
Alleles: Different molecular forms of a gene. Examples: Gene ‘a,’ Gene ‘b,’ Gene ‘C,’ Gene ‘D.’
Each gene has a specific locus on a chromosome.
Diploid cells (2n): Have pairs of genes on homologous chromosomes. Thus, diploid cells have two copies of every gene locus.
Haploid cells (n): Have only one copy of each type of chromosome, meaning haploid cells have only one copy of every gene locus.
Concept Clarifications
Diploid cells: Contain homologous chromosomes (two copies of each type).
Haploid cells: Contain one copy of each chromosome; thus, for any gene locus, only one allele is present.
Genotype and Phenotype Definitions
Genotype: The particular set of alleles for any one gene that an individual organism possesses (e.g., AA, Aa, aa).
An individual with two identical alleles of a gene is said to be homozygous for that gene (e.g., AA or aa).
An individual with non-identical alleles of a gene is said to be heterozygous for that gene (e.g., Aa, which is the same as aA).
Visual Representation of Genotypes
Homozygous (AA, aa) and heterozygous (Aa) depicted in genetic diagrams.
Determining Cell Types
Is this cell haploid or diploid?
A. Haploid
B. Diploid
C. Cannot be determined
What is the genotype of this cell?
A. AA
B. aa
C. Aa
Phenotype and Genotype Connection
An organism’s genotype determines its phenotype, which refers to an organism’s observable traits (e.g., Pp genotype resulting in purple flower phenotype).
Dominance and Recessiveness in Alleles
An allele is dominant if it “masks” or “hides” the phenotype of a recessive allele when paired with it.
Dominant alleles are represented by capital letters (P).
Recessive alleles are represented by lowercase letters (p).
Homozygous dominant = PP, homozygous recessive = pp, heterozygous = Pp.
Description of Cells Based on Genotypes
Which of the following properly describes this cell?
A. Homozygous dominant
B. Homozygous recessive
C. Heterozygous
D. Heterozygous dominant
E. Heterozygous recessive
Genetic Cross Terminology
(Genetic) Cross: Mating of two sexually reproducing individuals.
Hybrid: The heterozygote offspring of a cross between two individuals that breed true for different forms of a trait.
True-breeding: Organisms that are homozygous for the traits being considered. Example: Purple flower plant crossed with white flower plant typically leads to offspring that are hybrids in genotype.
P stands for parents, F for filial: This terminology is used to identify specific crosses and offspring generations.
P1: Parental cross, the original cross of parental organisms.
F1: First filial generation offspring of parental (P1) cross.
F2: Second filial generation offspring of the original parental (P1) cross.
Types of Genetic Crosses
Monohybrid cross: A genetic cross involving one trait (one gene locus).
Examples: Purple flower plant × white flower plant, green pea plant × yellow pea plant.
Dihybrid cross: Involves two traits (two different gene loci) and will be covered later.
Example of a Monohybrid Cross
True-breeding purple flower plant crossed with true-breeding white flower plant.
P1 cross (cross-fertilization) results in F1 generation offspring, all purple flower plants.
F1 cross (self-fertilization): Offspring = ¾ purple flower plants and ¼ white flower plants.
Meiosis and Genetic Prediction
The random sorting of chromosomes into gametes during meiosis aids in understanding genetic outcomes. The law of chance in fertilization plays a vital role in performing crosses.
Genotypic and Phenotypic Ratios in F2 Generation
When examining the F1 cross and its implications:
All progeny are purple flowers, while F2 generation results in:
Genotype ratio: 1 AA : 2 Aa : 1 aa.
Phenotype ratio: 3 purple : 1 white.
Use of Punnett Squares in Genetics
Probability: A measure of the likelihood that a specific outcome occurs.
Punnett Square: A grid utilized to calculate the probability of genotypes and phenotypes for offspring produced from a specified cross.
Constructing a Punnett Square Example
Example setup of male and female gametes showing genotypic ratios of:
AA = 1
Aa = 2
aa = 1
Flower phenotype outcomes translating to: Purple = 3, White = 1.
Mendel’s Four Laws of Inheritance
Formulated from his analysis of pea plant crosses:
Unit factors of inheritance exist in pairs.
For a pair of unit factors for a single trait, one unit factor is dominant and the other is recessive.
The paired unit factors segregate (separate) independently during gamete formation (Law of Segregation).
Different traits (involving different and distinct unit factor pairs) assort independently during gamete formation (Law of Independent Assortment).
These principles enabled Mendel to predict outcomes of genetic crosses effectively.
Connection Between Mendel's Laws and Meiosis
Mendel's laws illustrate how unit factors exist in pairs and how they segregate into daughter cells during gamete formation:
A = dominant alleles
a = recessive alleles
Demonstrated within the context of meiosis and subsequent predictions of inheritance.