L18: Patterns of Mendelian Inheritance

Question 1

What is the study of inheritance?

Answer 1

study of vertical transmission (from parent to offspring) and expression of genes (physical traits)

Question 2

What is your phenon? Which 4 factors impact your phenon?

Answer 2

• Your phenon (sum of all of your traits) is a consequence of:
• 1) the alleles you inherit
• 2) allele expression relationships
• 3) when genes are expressed
• 4) environment (cell→planet)

Question 3

You need to know these terms as they applied to our lesson of inheritance: homologues, autosomes, sex chromosomes, gene locus, allele, dominant allele, recessive allele, genotype, trait, character states, phenotype, homozygote(-ous), heterozygote(-ous), homozygous dominant, homozygous recessive, co-dominant heterozygote, mutation, polyallelic gene locus, gene pool, expression, expression relationship.

Answer 3

ok

Question 4

What is a Mendelian trait? What are examples of Mendelian traits? How can you recognize a Mendelian trait? Why do we focus on these kinds of traits even though not all of our traits our Mendelian?

Answer 4

A Mendelian trait is controlled by a single gene locus. (e.g., Rh+ or Rh– blood)

Mendelian traits are not necessarily the most common nor the
most important human traits. But they do enable us to make
predictions about potential offspring traits based on parental
traits because of their limited expression outcomes.

Question 5

What is a polygenic trait? What are examples of polygenic traits?

Answer 5

A polygenic trait is a trait controlled by several gene loci.
They vary continuously and express as several character
states (e.g., height, weight, color); too many to easily keep
track of to determine patterns of inheritance.

Question 6

What is a pleiotropic gene? What was the example of a potential pleiotropic gene locus used in class?

Answer 6

A pleiotropic gene is a gene locus whose expression impacts
more than a single trait (e.g., the ENPP-1 gene locus on
homologue 6 may underly Type-2 Diabetes AND obesity).

Question 7

For the previous, be able to recognize the relationship of the number of gene loci : number of traits. Mendelian are 1:1. What about the remaining two? (Use many for any number not 1.)

Answer 7

mendelian 1:1
polygenic many:1
pleiotropic 1:many

Question 8

What is an expression relationship? You will need to be able to discern among the 4 expression relationships, be able to identify which human blood traits are controlled by each, and how each expression relationship impacts the expression of a gene locus as a trait. For example, how does complete dominance differ from incomplete dominance, which blood traits exhibit complete/incomplete dominance, and how do these expression relationship govern the expression of the various character states of the blood traits?

Answer 8

Expression relationships are 4 models of the outcomes of
gene expression in diploid biota.

If we accept them to be true, they help us predict outcomes
in offspring phenotypes. (Models are not always true though.)

Complete Dominance: only the dominant allele is
expressed in heterozygotes (e.g., R r).

Co-Dominance: same as previous but 2 dominant alleles exist
in population resulting in some co-dominant heterozygotes
expressing both dominant phenotypes (e.g., IA IB).

Incomplete Dominance: heterozygotes have some degree
of expression intermediate between either homozygote.

Sex-linked: gene locus occurs on X or Y sex chromosomes so
expression outcome depends on sex. X-linked depends on how
many X chromosomes are inherited: Females = XX ; Males = XY

Question 9

What is Rh factor? On which homologue is the Rh factor gene locus found? Why is it important medically? What is an antigen?

Answer 9

Rh factor is a Red Blood Cell (RBC) glycoprotein found on homologue 1.

It acts as an antigen; a surface structure that can evoke an immune response.

Immune responses to Rh can complicate blood transfusion and fetal development for Rh- mothers with Rh+ mates

Question 10

Which expression relationship governs the Rh factor gene locus?

Answer 10

Rh is a Mendelian trait exhibiting complete dominance: only the dominant allele (R) is expressed in heterozygotes

Question 11

Which character state is associated with the expression of the dominant allele? Which character state is associated with the expression of the recessive allele? Which phenotype would be expressed by the following genotypes: RR, Rr, and rr? Which phenotype would be expressed by a the Rh factor gene locus that is: homozygous dominant, heterozygous, homozygous recessive?

Answer 11

Rh is a Mendelian trait exhibiting complete dominance: only the dominant allele (R) is expressed in heterozygotes

Genotypes —- &raquo_space; Phenotype
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
RR (homozygous dominant) —-> Rh+
Rr (heterozygous dominant) —-> Rh+
rr (homozygous recessive) —-> Rh-

Question 12

What is ABO blood type? On which homologue is the ABO blood type gene locus found? Why is it important medically? What is an antigen?

Answer 12

ABO Blood Type also refers to RBC glycoproteins: A and B.
They too act as an antigens and can evoke an immune response.
Immune responses to A and B antigens can complicate
blood transfusions.

Question 13

Which expression relationship governs the ABO blood type gene locus?

Answer 13

ABO exhibits co-dominance: same as complete dominance
but there are two dominant alleles (IA, IB) in the population

Question 14

Which character state is associated with the expression of the dominant allele A? Which character state is associated with the expression of the dominant allele B? Which character state is associated with the expression of the recessive allele? (ABO)

Answer 14

Genotypes —- &raquo_space; Phenotype
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
(homozygous dominant) IAIA, IAi —-> Type A
(homozygous dominant) IBIB, IBi —-> Type B
(homozygous recessive) ii —-> Type O (universal donor)
(co-dominant) IAIB —-> Type AB (universal recipient)

Question 15

Which phenotype would be expressed by the following genotypes: IAIA, IAi, IBIB, IBi, ii, and IAIB? Which phenotype would be expressed by a the ABO blood type gene locus that is: homozygous dominant, heterozygous, homozygous recessive, co-dominant? (ABO)

Answer 15

Genotypes —- &raquo_space; Phenotype
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
(homozygous dominant) IAIA, IAi —-> Type A
(homozygous dominant) IBIB, IBi —-> Type B
(homozygous recessive) ii —-> Type O (universal donor)
(co-dominant) IAIB —-> Type AB (universal recipient)

Question 16

Which blood type is the universal donor? Which blood type is the universal recipient? How do these designations relate to the RBC antigens present/absent?

Answer 16

No blood antigens = universal donor —> Type O
Co-expression of Type A and B = universal recipient —> Type AB

Question 17

What is Hemoglobin? How do B and S hemoglobin differ? On which homologue is the Hemoglobin (β-subunit) gene locus found? Why is S hemoglobin (HbS) important medically?

Answer 17

Hemoglobin (Hb) is a 4° protein that transports O2 in the blood.
β subunits form from expression of two alleles: HbB and HbS.
Expression of HbS causes Hb molecules within RBCs to polymerize elongating them resulting in sickled-shaped (undesired) RBCs.
HbB causes round (desired) RBCs.
found on homologue #11

Question 18

What is sickle-cell anemia? Which expression relationship governs the hemoglobin β-subunit gene locus?

Answer 18

Sickled RBCs results in sickle-cell anemia (SCA); poor O2 perfusion of tissues due to irregular shape of RBCs and their degradation by the immune system.

Hemoglobin exhibits incomplete dominance: the heterozygote has
some degree of expression intermediate of either homozygote.

Question 19

Which character state is associated with the expression of the HbB allele? Which character state is associated with the expression of the HbS? Which phenotype would be expressed by the following genotypes: HbB HbB, HbB HbS, and HbS and HbS?

Answer 19

character state:
HbB —-> round RBCs
HbS —-> sickle shape RBCs

Genotypes —- &raquo_space; Phenotype
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
HbB HbB —-> no SCA
HbB HbS —-> mild SCA
HbS HbS —-> full SCA

sickle cell anemia (SCA)

Question 20

What is Factor 8 (FVIII)? On which homologue is the FVIII gene locus found? Why is FVIII important medically?

Answer 20

FVIII is one of 13 blood clotting factors (proteins) needed to
clot blood. FVIII is made by an enzyme (itself a protein). found on the X sex chromosomes.

important because lack of can cause hemophilia

Question 21

What is Hemophilia A?

Answer 21

If one cannot make FVIII he/she cannot clot blood and has a
disease called Hemophilia A. Hemophiliacs must inject FVIII.

Question 22

Which expression relationship governs the FVIII gene locus? Which character state is associated with the expression of the dominant allele? Which character state is associated with the expression of the recessive allele?

Answer 22

Sex-linked X chromosome

character state:
dominant allele —-> normal FVIII enzyme, normal blood clotting
recessive allele —-> no blood clotting, hemophilia

Question 23

Which phenotype would be expressed by the following genotypes: XCL XCL, XCL Xcl, Xcl Xcl, XCL Y, and Xcl Y? Why is hemophilia A more common in males than females?

Answer 23

Genotypes —- &raquo_space; Phenotype
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
XCL XCL —-> clotting XX
XCL Xcl —-> clotting XX; carrier
Xcl Xcl —-> hemophiliac XX
XCL Y —-> clotting XY
Xcl Y —-> Hemophiliac XY

Men have a greater chance (1/2) at being hemophiliac compared to women (1/3 chance)
males have no opportunity to mask the effect of a recessive allele from one parent

Question 24

You will need to be able to work through inheritance problems similar to those in the inheritance handout based on the four gene loci that were the focus of this lesson.

Answer 24

ok

Question 25

What makes a recessive allele recessive? Are dominant alleles always the most frequent in a population? Why or why not?

Answer 25

Recessive alleles only show their effect if the individual has two copies of the allele (rr)

Whether an allele is dominant or not does not affect how common a trait is.
Describing a trait as dominant does not mean it is the most common; it means that it is expressed over the recessive trait.