Genetics: Part III Extending Mendel

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

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.

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.

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.

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.

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….

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

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

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.

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

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.

Calculating Crossover expected unexpected

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

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

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.

P Generation Red White CRCR CWCW Gametes CR CW F1 Generation Pink CRCW Figure 14.10-2 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.

P Generation Red White CRCR CWCW Gametes CR CW F1 Generation Pink CRCW Figure 14.10-3 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

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.

(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

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)

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.

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

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.

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.

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.

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

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