1. HOW INHERITED TRAITS ARE TRANSMITTED
Genetics = the study of the transmission of traits from one generation to the next.
Chapter 10

2. Genetics is the science of heredity.
Current theories of heredity can be traced back to Gregor Mendel.

3. Gregor Mendel Austrian monk with a strong mathematical background.
Interested in how certain traits were passed from parents to offspring.
Worked with pea plants ( ):
easy to grow & developed quickly
exhibited many traits that had 2 easily distinguishable forms
could manipulate fertilizations

4. Traits Mendel Studied
Mendel studied the transmission of these seven traits (seed form, seed color, pod form, pod color, flower position, seed coat color & stem length).
Notice that each trait has two easily distinguishable expressions (ex. seed color is either yellow or green)
We now know that each of these traits is controlled by a single gene (Mendel had no idea of what a gene was). The gene for each trait is located on a different chromosome.

5. Mendel’s Experimental Approach for Breeding Peas
Mendel could easily manipulate fertilizations.
In nature, pea plants usually self-fertilize because male (stamens) & female (carpel) plant parts are enclosed together.
Mendel would prevent self-fertilization in one flower by removing stamens. He would then fertilize the carpel of that flower by transferring pollen from a different pea plant.
He would later collect the cross-pollinated carpels, now pods filled with seeds.
He then planted the seeds. When the plants matured, he recorded his results.

6. Mendel’s observations:
Some pea plants were always “true breeding” (all offspring exhibit same trait as parents).
short x short  all short offspring
tall x tall  all tall offspring OR
 some tall, some short
One form of a trait tended to “mask” expression of the other form.
tall x short  all tall offspring OR
 some tall, some short

7. A tall x short mating never resulted in all short offspring.
Thus, the tall trait masked expression of the short trait.
Based upon these & subsequent observations, Mendel formulated the two basic laws of heredity:
Law of Segregation
Law of Independent Assortment
Mendel’s laws apply to all diploid species.
We shall discuss these 2 laws later.

8. A. Genetic Terminology
1. Chromosome - dark staining body in a cell’s nucleus; consists of DNA & proteins.
haploid cells - 1 set of chromosomes
diploid cells - 2 sets of chromosomes
2. Gene (elementen) - a sequence of DNA that codes for production of a specific protein.
Ex. stem length gene, seed color gene
Elementen is Mendel’s term for gene. Genes were not known to exist until the early 1900s. In 1909, Mendel’s elementen were renamed genes.
Traits controlled by a single gene are called Mendelian traits. All 7 traits that Mendel worked with are controlled by a single gene.

9. 3. Allele - an alternate form of a gene.
Ex. stem length gene has 2 alleles - tall allele & short allele
Dominant allele - allele that masks the expression of another allele.
Ex. tall allele (T)
Recessive allele - allele whose expression is masked by another allele.
Ex. short allele (t)
Dominant alleles are represented by uppercase letters.
Recessive alleles are represented by corresponding lowercase letters.

10. Diploid organisms possess 2 alleles for each gene.
2 dominant alleles for gene ‘A’
1 dominant & 1 recessive allele for gene ‘B’
A diploid cell has two alleles for every gene located on a homologous pair of chromosomes. [one allele from each of the organism’s parents]
2 recessive alleles for gene ‘D’

11. If the 2 alleles are identical, then organism is homozygous for that gene.
Ex. TT (homozygous dominant) tt (homozygous recessive)
If the 2 alleles are different, then organism is heterozygous for that gene.
Ex. Tt

12. 4. Genotype - the allele combination in an individual.
Ex. three possible genotypes for pea plant height: TT, Tt or tt
5. Phenotype - the observable expression of an allele combination.
Ex. two possible phenotypes for pea plant height: tall or short

13. If you know an individual’s phenotype, do you automatically know their genotype?
Yes, if the trait is recessive
short pea plants must be tt
round-seeded plants must be rr
No, if the trait is dominant
tall pea plants can be either TT or Tt
wrinkled-seeded plants can be either RR or Rr

14. B. Law of Segregation
The alleles of a gene separate during meiosis as chromosomes are packaged into gametes.
Law of Segregation is used to predict the proportions of progeny classes when only 1 trait is considered.
Diagram:
Germ cells of Parent 1 are heterozygous (Tt) for T gene. When they undergo meiosis, the T and t alleles will separate & be distributed to the resulting gametes. Notice that ½ of gametes posses the dominant allele (T), while the other ½ possess the recessive allele (t).
Germ cells of Parent 2 are homozygous dominant (TT) for the T gene. Thus, all of the gametes formed will possess the dominant allele (T).
At fertilization, gametes combine at random to form the individuals of a new generation.

15. We use a Punnett square to predict the outcome of a cross between two individuals.
Phenotypic ratio
3 tall : 1 short
Genotypic ratio
1 TT : 2 Tt : 1 tt
This is a monohybrid cross - cross between 2 individuals that are heterozygous for a the same gene.
Note: genotypic & phenotypic ratios are predicted values. If you actually did the cross, your actual results would vary slightly.

16. We use a test cross to determine an unknown genotype.
Test cross = a cross between an individual of unknown genotype and an individual that is homozygous recessive for the trait in question.
Homozygous recessive is the only genotype that can be identified by phenotype. Homozygous dominant & heterozygous genotypes have the same phenotype.

17. You are given a tall pea plant… determine it’s genotype.
Do a test cross: tt x unknown
if all offspring are tall, THEN…
tt TT
unknown genotype is TT Tt
Genotype of unknown is either TT or Tt.
if obtain both tall & short offspring, THEN…
tt Tt
unknown genotype is Tt
Tt tt

18. Mendelian Disorders in Humans:
Autosomal Recessive Traits:
located on non-sex chromosomes
parents are carriers or are affected
affected individuals are homozygous recessive
affects males & females
Ex. Albinism, Cystic fibrosis, Phenylketonuria, Sickle cell disease
Mendelian disorders in humans are rare. About 2,500 Mendelian disorders are known.
Traits carried on the X or Y chromosome are generally called sex-linked traits. These will be discussed in the next chapter.

19. Autosomal recessive traits can skip generations.

20. Autosomal Dominant Traits: located on non-sex chromosomes
at least one parent is affected
does not skip generations
affected individuals are homozygous dominant or heterozygous
affects males & females
Ex. Achondroplasia, Huntington disease, Lactose intolerance, Polydactyly
No such thing as a carrier of an autosomal dominant trait – heterozygotes are affected.

21. Autosomal dominant pedigree
Autosomal dominant traits do not skip generations.

22. C. Law of Independent Assortment
The segregation of one gene pair does not influence the segregation of another gene pair during meiosis.
Law of independent assortment is used to predict the proportions of the progeny classes when more than one trait (each controlled by a single gene) is considered.
NOTE: The law of independent assortment applies only if the genes in question are located on different chromosomes.
Figure - The independent assortment of genes carried on different chromosomes results from the random alignment of chromosome pairs during metaphase of meiosis I. Thus, an individual of genotype (RrYy; round, yellow seeds) produces 4 types of gametes (RY, Ry, rY, ry)

23. Punnett square of a dihybrid cross (RrYy x RrYy) yields a phenotypic ratio of (9:3:3:1):
9 round, yellow seeded offspring [RRYY, RrYY, RRYy & RrYy]
3 round, green seeded offspring [RRyy & Rryy]
3 wrinkled, yellow seeded offspring [rrYY & rrYy]
1 wrinkled, green seeded offspring [rryy]
Note: can make these predictions using the product rule which states that “the chance that two independent events will both occur equals the product of the individual chances that each event will occur”.
Ex. What is chance of obtaining a round, green seeded plant from two dihybrid parents (RrYy). A Punnett square for Rr x Rr shows that there is a 3/4 chance of obtaining a round seeded plant. A Punnett square for Yy x Yy shows that there is a 1/4 chance of obtaining a green seeded plant. Thus, there is a 3/16 (3/4 x 1/4) chance of obtaining a round, green seeded plant.

24. D. Factors Appearing to Violate Mendel’s Laws
1. Lethal Alleles - certain allele combination causes death of an entire phenotypic class very early in development.
Ex. hairless trait in dogs [homozygous dominant (HH) individuals die as embryos]
These factors appear to violate Mendel’s laws because they alter expected phenotypic ratios.

25. Hairless x hairless cross alters phenotypic ratios: obtain a 2:1 ratio rather than the expected 3:1 ratio.
To avoid dead embryos & stillbirths, breeders of Mexican hairless dogs will cross hairy x hairless rather than two hairless individuals.

26. 2. Multiple Alleles - gene exists as more than two alleles in the population.
Rabbit coat color gene has 4 alleles:
C, c, cch & ch
5 phenotypes
10 genotypes
Individuals can have only two alleles for a particular gene; however, several alleles can exist in the population.
Multiple alleles in varying combinations cause different variations of the phenotype.

27. 3. Epistasis - one gene masks the expression of another.
Ex. Bombay phenotype (H gene masks expression of I gene)
H gene is required to manufacture the glycoprotein that attaches A & B antigens to the surface of red blood cells. If individual is HH or Hh, the glycoprotein is produced and A & B antigens attach themselves appropriately.
If individual is hh, the glycoprotein is defective. A & B antigens do not attach to red blood cells eventhough the individual may have an ABO genotype indicating type A, B, or AB blood. Blood of an individual with hh genotype will always test as type O.
Note: hh genotype is extremely rare

28. 4. Incomplete Dominance - heterozygote expresses a phenotype intermediate between those of the two homozygotes.
Ex. snapdragon flower color
If cross Rr x Rr …
Note: phenotypic ratio is same as genotypic ratio.
Phenotypic ratio ->
1 red : 2 pink : 1 white
Genotypic ratio ->
1 RR : 2 Rr : 1 rr

29. 5. Codominance - heterozygote expresses a phenotype that is distinct from and not intermediate between those of the two homozygotes.
Ex. Human AB blood type
IA & IB alleles are codominant.
IAIB blood is different from IAIA or IBIB blood.
Human blood type is also an example of multiple alleles: IA, IB & i alleles.

30. E. Complex Traits
Traits that do not follow Mendel’s laws, but tend to “run in families”.
1. Polygenic Traits - determined by the combined effect of more than one gene.
Ex. height, eye color & skin color

31. Human Skin Color
The genes that control skin color are neither dominant nor recessive with respect to each other. Their effect is additive. The greater the total number of dominant alleles, the darker the skin.

32. 2. Multifactorial Traits - determined by the combined effect of one or more genes plus the environment.
Ex. heart disease, body weight, intelligence
Heart disease is determined by the combined effect of genes (control blood pressure, clotting ability, lipid levels in blood) and the environment (diet, exercise habits, stress).