1. Genetics: Part III
Extending Mendel

2. Hypothesis of dependent assortment
Figure 14.8
EXPERIMENT
YYRR
P Generation
yyrr
Gametes YR yr
F1 Generation
YyRr
Predictions
Hypothesis of dependent assortment
Hypothesis of independent assortment
Sperm
Predicted offspring of F2 generation or
1/4 YR
1/4 Yr
1/4 yR
1/4 yr
Sperm
1/2 YR
1/2 yr
1/4 YR
YYRR
YYRr
YyRR
YyRr
1/2 YR
YYRR
YyRr
1/4 Yr
Eggs
YYRr
YYrr
YyRr
Yyrr
Eggs
As we get started today, think about the laws of independent assortment and segregation. Take a few minutes to explain these two laws to your partner.
Now that these two are fresh in you mind, consider the following:
1/2 yr
YyRr
yyrr
1/4 yR
YyRR
YyRr
yyRR
yyRr
3/4
1/4
1/4 yr
Phenotypic ratio 3:1
YyRr
Yyrr
yyRr
yyrr
9/16
3/16
3/16
1/16
Phenotypic ratio 9:3:3:1
RESULTS
315
108
101 32
Phenotypic ratio approximately 9:3:3:1

3. Which of these genes are most likely to segregate as a unit?
The image shown represents the location of 4 genes on human chromosome number 15 as identified during the human genome project. Which genes are most likely to segregate as a unit? Talk to your partner. Be ready to explain your reasoning.
Brown eye and brown hair color are most likely to segregate as a unit. You may remember from discussions of meiosis, that genes that are close together are not as likely to get separated during meiosis. Those that are further apart are more likely to get separated. These two, brown eye, brown hair are relatively close together and as a result are most likely to remain on the same chromosome during segregation events.

4. Which of these genes are most likely to segregate as a unit?
The image shown represents the location of 4 genes on human chromosome number 15 as identified during the human genome project. Which genes are most likely to segregate as a unit? Talk to your partner. Be ready to explain your reasoning.
Brown eye and brown hair color are most likely to segregate as a unit. You may remember from discussions of meiosis, that genes that are close together are not as likely to get separated during meiosis. Those that are further apart are more likely to get separated. These two, brown eye, brown hair are relatively close together and as a result are most likely to remain on the same chromosome during segregation events.

5. Curriculum Framework
3.A.3.b.2 Genes that are adjacent and close to each other on the same chromosome tend to move as a unit; the probability that they will segregate as a unit is a function of the distance between them.
Although Mendel knew traits would segregate, our current understanding of the structure of chromosomes helps us to explain why that is not always the case. As we move through this session we will look at additional genetic principles that extend our understanding beyond that of Mendelian genetics.

6. In Drosophila…..
Traits for wings, leg length, and eye color are carried on the same chromosome. These genes are linked together on the same chromosome and will sort into the same gamete.
The genome of Drosophila melanogaster is composed of 4 pairs of chromosomes. So it is not surprising that they express linked traits. Traits for wings, leg length, and eye color are carried on the same chromosome. These genes are linked together on the same chromosome and will sort into the same gamete.

7. This linkage is revealed when a test cross if performed.
If a homozygous P generation wild type (gray with normal wings) is crossed with homozygous recessive (black vestigial winged) mutant….

8. Then the offspring will be heterozygous for gray body and normal wings.
If the F1s are test crossed with the homozygous recessive mutant

9. Then one would expect to have equal numbers gray normal, black vestigial, gray vestigial and black normal offspring.

10. If the genes are linked and travel as one, then you would expect to get only gray, normal and black vestigial.
Notice that the results however show a few gray vestigial and a few black normal , both of which would be unexpected if linked and higher in number if not linked. How can we explain the results? Crossing over has occurred.

11. Homozygous gray body, normal wing crossed with homozygous black body vestigial wing

14. Crossover Value
Crossover value gives a glimpse of the physical location of the genes on the chromosome. The higher the frequency of crossing over, the farther apart the genes are on the chromosome.
The crossover value represents the frequency of recombination which is converted to map units, with one map unit being equal to a 1 percent crossover rate.

15. Calculating Crossover
expected
unexpected

16. Extending Mendelian Genetics for a Single Gene
Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations:
When alleles are not completely dominant or recessive
When a gene has more than two alleles
When a gene produces multiple phenotypes

17. Degrees of Dominance
Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical
In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways

18. P Generation Red White CRCR CWCW Gametes CR CW Figure 14.10-1
Flower color exhibits Incomplete dominance in snapdragons. Notice here in the P1 Generation, a red flowering plant is crossed with a white flowering plant.

19. P Generation Red White CRCR CWCW Gametes CR CW F1 Generation Pink CRCW
Figure
P Generation
Red
White
CRCR
CWCW
Gametes CR CW
F1 Generation
Pink
CRCW
Gametes
1/2 CR
1/2 CW
The resulting F1 generation produces all pink flowers, showing a third phenotype.

20. P Generation Red White CRCR CWCW Gametes CR CW F1 Generation Pink CRCW
Figure
P Generation
Red
White
CRCR
CWCW
Gametes CR CW
F1 Generation
Pink
CRCW
Gametes
1/2 CR
1/2 CW
Sperm
If the F1 plants are mated, they can produce offspring that are red, pink or white. The probability occurs in a 1:2:1 ratio which is typical of traits exhibiting incomplete dominance.
F2 Generation
1/2 CR
1/2 CW
1/2 CR
CRCR
CRCW
Eggs
1/2 CW
CRCW
CWCW

21. Multiple Alleles
Human blood types are example of a trait governed by multiple alleles that express codominance. The four different types ( A, B, AB, and O) are named based on the enzyme that attaches a specific type of carbohydrate to the blood cells.

22. (a) The three alleles for the ABO blood groups and their carbohydrates
Figure 14.11
(a) The three alleles for the ABO blood groups and their carbohydrates
Allele IA IB i
Carbohydrate A B
none
(b) Blood group genotypes and phenotypes
Genotype
IAIA or IAi
IBIB or IBi
IAIB ii
The four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i.
The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither
Red blood cell appearance
Phenotype (blood group) A B AB O

23. Key to blood type genotypes
To solve genetics problems involving blood types, you need to keep the key in mind. There are two genotypes for both type A and type B blood. There is only one genotype for AB (AB) and one genotype of type O (ii)

24. Solve It
Sara, who is type O, claims that Ira is the father of her baby. The baby has type O blood. Ira is blood type B. Is it possible that Ira is the father?
Understanding the key to blood types will allow you to readily solve problems like this one. Take a moment to read and solve the problem. Draw a Punnett square to support your work.

25. Solve It
So yes, If Ira is heterozygous for blood type B he can produce a type O child with Sara.

26. Curriculum Framework
3.A.4: The inheritance pattern of many traits cannot be explained by simple Mendelian genetics.
Many traits are the product of multiple genes and/or physiological processes.
1. Patterns of inheritance of many traits do not follow ratios predicted by Mendel’s laws and can be identified by quantitative analysis, where observed phenotypic ratios statistically differ from the predicted ratios.

27. Polygenic Inheritance
Another inheritance pattern that extends beyond Mendelian genetics is called polygenic inheritance. Human skin comes in a wide variety of “colors” . This is an example of a trait with Quantitative characters which are those that vary in the population along a continuum.

28. Number of dark-skin alleles: 1 2 3 4 5 6 1/64 6/64 15/64 20/64 Eggs
Figure 14.13
Number of dark-skin alleles: 1 2 3 4 5 6
1/64
6/64
15/64
20/64
Eggs
Sperm
Phenotypes:
1/8
AaBbCc
Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype
Skin color in humans is an example of polygenic inheritance. The higher the number of dark skin alleles, the darker the phenotype.

29. Influence of Environment
Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype
The norm of reaction is the phenotypic range of a genotype influenced by the environment
For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity

30. Created by:
Debra Richards
Coordinator of K-12 Science Programs
Bryan ISD
Bryan, TX