1. Genetics (10.2, 10.3, Ch.11)
SB2. Students will analyze how biological traits are passed on to successive generations.
(c) Using Mendel’s laws, explain the role of meiosis in reproductive variability.
(e) Compare the advantages of sexual reproduction and asexual reproduction in different situations.

2. Some Needed Vocabulary
Heredity
Passing on of characteristics from parent to offspring
Traits
Characteristics that are inherited
Genetics
The study of Heredity
Gametes
Sex cells
Sperm = male gamete
Egg = female gamete

3. Gregor Mendel A monk who studied heredity using garden pea plants
Why garden peas?
They reproduce sexually
They have both male & female gametes on the same plant
What did he do?
He transferred pollen from one plant to another with different traits
This is called “making a cross”
Mendel first “crossed” tall plants with short plants

4. Mendel’s Laws Law of segregation Law of independent assortment
The two alleles for each trait must separate when gametes are formed
A parent will pass down AT RANDOM, only one allele for each trait
Law of independent assortment
Genes for different traits are inherited independently of one another

5. Mendel’s Monohybrid crosses
Focuses on one (mono) trait
1st generation
Crossed a tall pea plant with a short pea plant
All of the offspring were tall

6. Mendel’s Monohybrid crosses
2nd generation
Crossing the offspring of the 1st generation
¾ of the offspring were tall; ¼ of the offspring were short
3:1 ratio (tall to short)

7. Mendel’s Findings
Mendel found that each chromosome has two factors for each traits… called alleles
Alleles = forms of genes
Types of alleles
Dominant
The allele that has the ability to mask the other
Represented with a capital letter (A)
Recessive
The allele that will be masked by the dominant allele… it will be there, but will not be expressed
Represented with a lowercase letter (a)

8. Phenotype vs. Genotype Phenotype Genotype
The way that an offspring looks; how the trait appears
Tall, short
Genotype
The offspring’s genetic combination
TT, Tt, tt

9. Homozygous vs. Heterozygous
Homozygous (“homo” means “same”)
Having two identical alleles for a trait
TT, tt
Heterozygous (“hetero” means “different”)
Having two different alleles for a trait Tt

10. Punnett square
Short way to find the expected proportions of possible genotypes in the offspring of a testcross

11. Monohybrid cross vs. dihybrid cross
Involves the cross of ONE trait
Uses a 4 block punnett square
Dihybrid cross
Involves the cross of TWO traits
Uses a 16 block punnett square
“F.O.I.L.” to produce gametes

12. Complete Dominance
Inheritance characterized by an allele that is fully expressed in the phenotype of a heterozygote and that masks the phenotypic expression of the recessive allele
Example:
In humans, dimples are dominant over no dimples. What is the probability of producing a dimpled child from two heterozygous dimpled parents?
Complete dominance
Inheritance characterized by an allele that is fully expressed in the phenotype of a heterozygote and that masks the phenotypic expression of the recessive allele
Incomplete dominance
Pattern of inheritance in which the dominant phenotype is not fully expressed in the heterozygote, resulting in a phenotype intermediate between the homozygote dominant and homozygous recessive
Codominance
Inheritance characterized by full expression of both alleles in the heterozygote

13. Exceptions to Mendel’s Law
Mendel’s traits showed two distinct forms
Most genes do not exhibit simple inheritance
Genotypic ratios persist but phenotypic ratios may vary because of interactions between
Alleles
Other genes
Segregation of genes on the same chromosome
Environment

14. Lethal Alleles Some allele combinations are lethal
Must be taken into consideration when completing punnett squares

15. Incomplete Dominance Example:
In snapdragons (a type of plant), pink and white petal colors show incomplete dominance. What is the likelihood of producing a red flower if two pink flowers are crossed?
Pattern of inheritance in which the dominant phenotype is not fully expressed in the heterozygote, resulting in a phenotype intermediate between the homozygote dominant and homozygous recessive
Complete dominance
Inheritance characterized by an allele that is fully expressed in the phenotype of a heterozygote and that masks the phenotypic expression of the recessive allele
Incomplete dominance
Pattern of inheritance in which the dominant phenotype is not fully expressed in the heterozygote, resulting in a phenotype intermediate between the homozygote dominant and homozygous recessive
Codominance
Inheritance characterized by full expression of both alleles in the heterozygote

16. Codominance
Example:
In birds, specifically chickens, black and white feather color is expressed in a codominant fashion. What is the likelihood of producing a black bird if a white bird and black bird are crossed?
Inheritance characterized by full expression of BOTH alleles in the heterozygote
Complete dominance
Inheritance characterized by an allele that is fully expressed in the phenotype of a heterozygote and that masks the phenotypic expression of the recessive allele
Incomplete dominance
Pattern of inheritance in which the dominant phenotype is not fully expressed in the heterozygote, resulting in a phenotype intermediate between the homozygote dominant and homozygous recessive
Codominance
Inheritance characterized by full expression of both alleles in the heterozygote

17. Multiple Alleles
Some genes may have more than just two alternative forms of a gene
The inheritance of the ABO blood groups is an example of a locus with three alleles

18. Multiple Alleles Example:
In humans, one of the ways blood types are determined is with a trait controlled by multiple alleles. What is the likelihood of producing a child with type O blood from two heterozygous parents one with type A blood and one with type B blood?

19. Epistasis
Interaction between two non-allelic genes in which one modifies the phenotypic expression of another
In horses, brown coat color (B) is dominant over tan (b).
Gene expression is dependent on a second gene that controls the deposition of pigment in hair.
The dominant gene (C) codes for the presence of pigment in hair, whereas the recessive gene (c) codes for the absence of pigment.
Interaction between two non-allelic genes in which one modifies the phenotypic expression of another
For example:
The gene for pigment deposition (C) is epistatic to the gene for pigment (melanin) production.
Whether the pigment can be deposited in the fur determines whether the coat color can be expressed
Homozygous recessive for pigment deposition (cc) will result in an albino mouse regardless of the genotype at the black/brown locus (BB, Bb, or bb)
CC, Cc = Melanin deposition
cc = albino

20. Polygenic Inheritance
Mode of inheritance in which the additive effect of two or more genes determines a single phenotypic character
For example, skin pigmentation in humans appears to be controlled by at least three separately inherited genes.
Three genes with the dark skin allele (A, B, C) contribute one unit of darkness to the phenotype. These alleles are incompletely dominant over the other alleles (a, b, c)
An AABBCC person would be very dark & an aabbcc person would be very light. AaBbCc = intermediate shade

21. Sex-Linked Alleles Example:
Controlled by genes located on sex chromosomes (XX or XY)
Usually carried on X chromosome
Since females are XX, they are usually carriers of the trait
Since males are XY, they have a higher tendency for inheritance and EXPRESSION of the trait
Example:
In humans, there is a genetic condition known as Hemophilia that is controlled by sex-linked inheritance. What is the likelihood of having a female carrier if a hemophiliac dad and a normal mom are crossed?