Download Introduction to Genetics and Heredity

Introduction to Genetics and Heredity
Although these dogs
have similar
characteristics they
are each unique!
I. Early Ideas About Heredity
A. The Theory of Blending Inheritance
• 
Each parent contributes factors that blend in their offspring
- ex. A short plant crossed with a tall plant would produce a medium sized plant.
B. Gregor Mendel
• 
Born in 1822 in Czech Republic
• 
Worked as a teacher and performed research on the heredity of pea plants
C. Genetics
• 
The scientific study of heredity
D. Mendel’s Experiment
1.  Procedure:
a.  Cross-pollinate purebred pea plants
b.  Use pea plants with different characteristics for the same trait
- Traits include: seed shape and color, flower position, plant height etc.
D. Mendel’s Experiment
2. Results:
a.  The F1 (offspring) generation had the traits of only one
of the parents
b.  The P1 (parental) generation’s traits did not blend
C.  Mendel’s Experiment
3.  Conclusions:
a.  Individual factors, which do not blend, control each trait of a living thing. These factors are called genes.
b.  The different forms of a gene are called alleles.
•  For example, the gene for plant height occurs in tall
and short form.
c.  Some alleles are dominant, while others are recessive.
•  The effects of a dominant allele are seen even if a recessive allele is present.
•  The effects of a recessive allele are seen only if a dominant allele is not present.
II. Using Genetic Vocabulary
A.  Defining Terms:
1. DNA is organized into chromosomes. There are 23 pairs of chromosomes (46 total) in all human cells (except sex cells).
2. Genes are small segments of DNA present on chromosomes that code for a particular protein. Multiple genes are found on one
chromosome.
- The estimated # of genes in the human genome is between 30,000 and
35,000
3. Genes code for proteins, ultimately resulting in the expression
of specific traits (characteristics).
II. Using Genetic Vocabulary Cont.
4. Genes come in different forms called alleles. Alleles are either
dominant (A) or recessive (a).
5. Organisms that have two identical alleles for a particular trait
are said to be homozygous (AA) or (aa).
6. Organisms that have two different alleles for the same trait are
heterozygous (Aa).
7. The genotype (genetic make-up) for a particular trait determines
the phenotype (physical characteristic).
III. A Simple Example
The organisms in our fictional example have one pair of chromosomes per body cell. A
gene on the chromosome codes for either dark (D) or light (d) flower color.
P1 (parental) Generation
Female:
Male:
Genotype:
dd
Homozygous recessive
Phenotype: Light flower color
DD
Homozygous dominant
Dark flower color
A cross between two parents with different forms of a gene is called a hybrid.
To prepare for mating, special cells called gametes must form. The process used to
make gametes is called meiosis. Gamete cells are haploid (containing only one
homologous chromosome per pair).
P1 Generation
Male:
Pollen
(sperm)
Female:
d
d
d
d
D
D
Ovule
(egg)
D
D
Note: In animals, 3 of the egg cells (polar bodies) will degenerate!
When organisms “mate”, 1 sperm cell (or pollen grain) from dad will randomly
combine with an egg cell (or ovule) from mom. These combined gametes will produce a
diploid (2N) offspring with 2 complete sets of chromosomes.
In this example, all of the pollen cells contain the recessive allele (d) for flower color and
the ovule cell contains the dominant allele (D) for flower color
Male gamete:
pollen or sperm
Female gamete:
ovule or egg
d
Fertilization
F1 (offspring) generation
D
All offspring will be Dd with Dark
flowers
Dd =
Genotype
heterozygous
Dark flower color = Phenotype
IV. Punnett Squares
D
d
D = dark flowers
d = light flowers
D
DD
Dd
d
Dd
dd
A Punnett square is used to predict the probability
Of producing offspring with certain characteristics.
The probability of two heterozygous dark flowered plants
Producing a dark flowered plant is 3/4 while the probability
Of producing a light colored plant is 1/4.
V. Special Cases of Dominant and Recessive
Incomplete Dominance: a genetic cross where one allele is
Not completely dominant over another. Example - red and
White flowers combine to produce pink flowers
Codominance: a genetic cross where both alleles show up
In the phenotype for the organism. Example - red and
White flowers combine to produce speckled red and white
Flowers
Multiple Alleles: Characteristics that have more than one
Possible allele. Example - the gene for human eye color
Comes in many different forms.
Polygenic Traits: Traits that are controlled by two or more
Genes. Example - about four different genes control human
Skin color