Unit 18: Inheritance

3.4.U1: Mendel discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed.

Describe Mendel’s pea plant experiments.

Through selective breeding of pea plants, Mendel discovered that certain traits show up in offspring without blending of the parent characteristics. Mendel observed seven traits: flower color, stem length, seed color, pod color, flower position, seed shape. Mendel concluded: 1) genetic units of inheritance are passed from parent to offspring.

2) the offspring inherits one “unit” from each parent for each trait

3) the “unit” may be masked or hidden (recessive) in an individual but can still be passed on the to the next generation

3.4.NOS: Making quantitative measurements with replicates to ensure reliability, Mendel’s genetic crosses with peas plants generated numerical data.

Outline why Mendel’s success is attributed to his use of pea plants.

Mendel’s use of peas allowed for the observation of easily distinguishable characteristics (i.e. green or yellow pea pods) Also, the peas are able to reproduce quickly allowing for man generations of data to be collected. Lastly, the reproduction can be controlled, so Mendel knew exactly which two parent plants were being bed (either cross or self pollination).

List three biological research methods pioneered by Mendel.

1) Large number of replicates to demonstrate reliability of results2) Repeats of whole experiments (Mendel had seven different crosses as experiments)3) Obtaining quantitative results, not only descriptions

3.4.U2: Gametes are haploid so contain only one allele of each gene.

Define gamete and zygote.

Gamete: a reproductive cell (egg or sperm) that fuses with another gamete of the opposite sex during fertilization.

Zygote: a diploid cell resulting from the fusion of two haploid gametes; the fertilized egg

State two similarities and two differences between male and female gametes

Both egg and sperm are haploid (have 23 chromosomes in humans) cells produced through meiosis.

The egg and sperm are very different in size and shape. Eggs are large cells where as sperm are much smaller. Sperm have flagella, eggs do not.

3.4.U4: Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles

Outline the possible combination of alleles in a diploid zygote for a gene with two alleles.

Alleles are variations of a single gene. Although there can be (and often are) multiple alleles for a gene in the population, any single individual can only have a maximum of two alleles of a gene; one allele on each chromosome of a homologous pair.

For a gene with two alleles, the zygote could be either: 1) homozygous dominant (two copies of the dominant allele. 2) heterozygous (one cope of the dominant allele and one copy of the recessive allele) 3) homozygous recessive (two copies of the recessive allele

Outline the possible combination of alleles in a diploid zygote for a gene with three alleles.

Many genes have multiple alleles, within the population, but within a diploid individual they can have a max of two alleles.

Within this population there may be three or more alleles, versions of the gene.

Within this individual there can only be a maximum of two alleles, versions of the gene.

For example, in ABO blood typing there are three common alleles for “isoagglutinogen gene”: IA, IB, and i

An individual may only have any combination of two of the alleles.

3.4.S1: Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.

Define monohybrid, true breeding, P and F1.

Monohybrid: a genetic cross between two individuals tracking one gene of interest.

True breeding: organisms that have been bred to have a homozygous genotype

P generation: the parent generation of a genetic cross

F1 generation: the offspring of the P generation cross “first filial”

Determine possible alleles present in gametes given parent genotypes.

Construct Punnett grids for single gene crosses to predict the offspring genotype and phenotype ratios.

F1 generation: ¼ SS Smooth, ½ Ss Smooth, ¼ ss wrinkled

3.4.U5: Dominant alleles mask the effect of recessive alleles but codominant alleles have joint effects.

Define dominant allele and recessive allele.

Different versions of a gene are called alleles. Dominant alleles show their effect even if the individual is heterozygous they can mask the

presence of another allele. Recessive alleles only show their effect if the individual has two copies (homozygous) otherwise

their presence is masked by a dominant allele.

State an example of a dominant and recessive allele found in pea plants.

State the usual cause of one allele being dominant over another.

The cause of allele dominance is complex and can vary between genes. However, in general:

Dominant allele codes for functioning proteins Recessive allele codes for less or non-functioning protein

Sometimes the recessive allele is the “normal” or “healthy” version of the gene.

Define codominant alleles.

With codominant alleles, both alleles are expressed equally; there isn’t masking of a recessive allele by a dominant allele.

Using the correct notation, outline an example of codominant alleles.

Since there isn’t a true dominant allele, a lower case letter is not used when alleles are codominant. Rather, two different capital letters are used and placed as a superscript to a common letter that represents the name of the gene. For example, type A and type B alleles of the isoaglutinogen blood type gene.

3.4.U9: Many genetic diseases have been identified in humans but most are very rare.

15. List five example genetic diseases.

Cystic fibrosis (autosomal recessive) Hemophilia (sex-linked recessive) Huntington’s disease (autosomal dominant) Phenylketonuria; PKU ( autosomal recessive) Red-Green color blindness (sex-linked recessive)

16. Explain why most genetic diseases are rare in a population.

Most genetic diseases are caused by alleles that are rare in a population. The chance of inheriting one allele for an autosomal dominant disease is 50% IF you have a heterozygous parent with the disease. In the case of an autosomal recessive disease, a person must inherit two copies of a rare allele, one from each parent.

3.4.U6: Many genetic diseases in human are due to recessive alleles of autosomal genes.

17. Define “carrier” as related to genetic diseases.

A genetic carrier is a person (or other organism) that has inherited a recessive allele of a gene but does not display the symptoms of the disease because they also have a dominant (normal functioning) allele. They must be heterozygous.

18. Explain why genetic diseases usually appear unexpectedly in a population.

Recessive alleles can be masked by dominant alleles for generations. If two carriers who show no disease symptoms mate, there is a ¼ chance of the offspring showing disease characteristics.

D.1.A2: Cause and treatment of phenylketonuria.

Outline the genetic cause of phenylketonuria.

PKU is an autosomal recessive allele. The gene for PKU is located on chromosome 12. If a child inherits two of the recessive alleles from parents, they will have the disease

PP = no disease Pp = no disease, carrier pp = PKU disorder

List consequences of phenylketonuria if untreated.

The PKU gene codes for an enzyme needed to break down phenylalanine (an amino acid). Without the enzyme, phenylalanine will build up. Without treatment, PKU symptoms include

Intellectual disability Delayed development Bone weakness Rashes Seizures

People with PKU disorder must eat a very low protein diet to limit phenylalanine intake.

3.4.U7: Some genetic diseases are sex-linked and some are due to dominant or codominant alleles.

Describe why it is not possible to be a carrier of a disease caused by a dominant allele.

A carrier is an individual with a heterozygous genotype, carrying the disease allele but not showing the disease phenotype.

If the disease is due to a dominant allele, the individual will show the disease phenotype when heterozygous.

Outline inheritance patterns of genetic diseases caused by dominant alleles.

Only one copy of the dominant disease allele is needed for the individual to express the disease phenotype.

If the parent is homozygous dominant there is a 100% chance the offspring will inherit the allele. If the parent is heterozygous, there is a 50% chance the offspring will inherit the allele.

All affected individuals have at least one parent with the disease. (example: Huntington Disease)

Explain sickle cell anemia as an example of a genetic disease caused by codominant alleles.

Sickle cell anemia is a disease where red blood cells (RBC) become thin and elongated.

If a person has one copy of the sickle cell allele, half of their red blood cells will be misshapen. In this way, the alleles are codominant, since both normal and sickled shapes are seen.

CODOMINANT: when both alleles of a gene in a heterozygote are expressed

In a heterozygote the 50% of the cells that are normal shape are able to carry enough oxygen so no symptoms of the disease are shown. To show symptoms of sickle cell anemia, the individual is homozygous recessive, so 100% of their RBC cells are sickled.

Define sex linkage

“Sex linkage” is for genes located on the sex chromosomes. The gene’s expression, inheritance pattern, and effect on the phenotype will differ between males and females.

3.4.A3: Inheritance of cystic fibrosis and Huntington’s disease.

Describe the relationship between the genetic cause of cystic fibrosis and the symptoms of the disease.

A mutation in the CFTR gene causes cystic fibrosis. This gene codes for a transport protein that regulates the movement of salt and water in and out of cells. If the gene is mutated, the resulting protein has a slight change in shape that limits or prevents its proper function. This causes sticky mucus that clogs the tubes that carry air in and out of the lungs, resulting in symptoms such as:

Lung infections Breathlessness Wheezing Persistent cough

Outline the inheritance pattern of cystic fibrosis.

CF is an autosomal recessive pattern

Outline the inheritance pattern of Huntington’s disease.

Huntington’s is an autosomal dominant pattern

List effects of Huntington’s disease on an affected individual.

Symptoms of Huntington’s disease develop between ages of 30 and 50

Uncontrolled movements Decline in cognition Loss of memory Changes in mood

3.4.A1: Inheritance of ABO blood groups.

Describe ABO blood groups as an example of complete dominance and codominance.

The ABO blood groups are determined by a single gene (I). The gene codes for an enzyme protein that modifies the carbohydrate molecule attached to an antigen protein on the surface of red blood cells. The “I” gene has three alleles:

IA: codes for an enzyme that attaches galactosamine to RBC antigens

IB: codes for an enzyme that attaches galactose to RBCi: codes for inactive proteins

Alleles IA and IB are completely dominant over allele i.

Outline the differences in glycoproteins present in people with different blood types.

A glycoprotein is a protein with a carbohydrate group attached. Glycoproteins are often located on the surface of cell membranes, for example on red blood cells (see above picture)

3.4.U8: The pattern of inheritance is different with sex-linked genes due to their location on sex chromosomes.

Outline Thomas Morgan’s elucidation of sex linked genes with Drosophila.

Thomas Hunt Morgan studied genetics of fruit fly, drosophilia. He is credited with the discovery of sex linked traits; traits that appear to associate differently in males and females.

Flies normally have red eyes, but there was a mutant male with white eyes.

All offspring were red eyed = red eyes are dominant

Only males have white eyes, suggesting white is carried on X chromosome.

Use correct notation for sex linked genes.

With sex-linked traits, the X and Y chromosome are shown with symbols for dominant and recessive alleles written as superscripts on the chromosome. For example:

XR = red eye Xw = white eye Y = no superscript because the gene is not on the Y chromosome

Describe the pattern of inheritance for sex linked genes.

There are many X-linked genes, most of which code for something other than anatomical traits. In the vast majority of cases, the dominant allele codes for the normal condition and the recessive allele codes for the disease or disorder.

Because men only have one X chromosome, genes on the X are expressed in the male phenotype. In females, a recessive allele on one X chromosome can be masked by a dominant allele on the other X chromosome. This explains why females can be carriers and why X-linked diseases/disorders are more observed in males.

Construct Punnett grids for sex linked crosses to predict the offspring genotype and phenotype ratios.

3.4.A2: Red-green color blindness and hemophilia as examples of sex-linked inheritance.

Describe the cause and effect of red-green color blindness.

Red-Green color blindness is caused by a sex-linked recessive allele of a gene that codes for a protein (opsin) that is sensitive to particular wavelengths of light. The mutated allele causes red-green color vision defects. [Opsin is found on cone cells in the retina of the eyes]

Explain inheritance patterns of red-green color blindness.

Describe the cause and effect of hemophilia.

Hemophilia is caused by a mutated allele of a gene that codes for a protein that is essential in the blood clotting process. Without proper clotting, there can be excessive bleeding.

Explain inheritance patterns of hemophilia.

3.4.S3: Analysis of pedigree charts to deduce the pattern of inheritance of genetic diseases

Outline the conventions for constructing pedigree charts.

A pedigree chart is a diagram that shows the occurrence of a phenotype in generations of a family.

Deduce inheritance patterns given a pedigree chart.

Autosomal dominant:

Appears equally in males and females Does not skip generations Affected offspring have an affected parent

Autosomal Recessive:

appears equally in males and females can skip generations

X-linked Recessive

More males than females Can skip generations Mothers of affected sons are carriers