IB DP Biology Topic 3: Genetics 3.4 Inheritance Study Notes

​Genetic inheritance is a basic principle of genetics. It explains how characteristics are passed from one generation to the next.

Genetic inheritance occurs due to genetic material in the form of DNA being passed from parents to their offspring. When organisms reproduce, all the information for growth, survival and reproduction for the next generation is found in the DNA passed down from the parent generation.

Much of our understanding of inheritance began with the work of a monk by the name of Gregor Mendel. His experiments and ‘Laws of Inheritance’ provide the foundations for modern genetic.

This unit will take 4 school days

Essential idea:

• The inheritance of genes follows patterns.

Nature of science:

• Making quantitative measurements with replicates to ensure reliability. Mendel’s genetic crosses with pea plants generated numerical data. (3.2)
  • Outline why Mendel’s success is attributed to his use of pea plants.
  • Explain discoveries made by Thomas Andrew Knight in relation to current understanding of genetics.
  • List three biological research methods pioneered by Mendel.

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.​

Gregor Mendel was an Austrian monk who developed the principles of inheritance by performing experiments on pea plants. Mendel discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed

• Some characteristics cannot be inherited
• Mendel’s experiment used different types of pea plants, each of which reliably had the same characters when grown on its own
• Replicates and reliability in Mendel’s experiments
• Making quantitative measurements with replicates to ensure reliability: Mendel’s genetic crosses with pea plants generated numerical data
• Male and female parents contribute equally to offspring

Mendel drew the following conclusions:

• Organisms have discrete factors that determine its features (these ‘factors’ are now recognised as genes)
• Furthermore, organisms possess two versions of each factor (these ‘versions’ are now recognised as alleles)
• Each gamete contains only one version of each factor (sex cells are now recognised to be haploid)
• Parents contribute equally to the inheritance of offspring as a result of the fusion between randomly selected egg and sperm
• For each factor, one version is dominant over another and will be completely expressed if present

Certain rules can be established:

• Law of Segregation: When gametes form, alleles are separated so that each gamete carries only one allele for each gene
• Law of Independent Assortment: The segregation of alleles for one gene occurs independently to that of any other gene
• Principle of Dominance: Recessive alleles will be masked by dominant alleles

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

• Define gamete and zygote.
• State two similarities and two differences between male and female gametes

Gametes are haploid sex cells formed by the process of meiosis – males produce sperm and females produce ova

• During meiosis I, homologous chromosomes are separated into different nuclei prior to cell division
• As homologous chromosomes carry the same genes, segregation of the chromosomes also separates the allele pairs
• Consequently, as gametes contain only one copy of each chromosome they therefore carry only one allele of each gene
• Cells that fuse together to produce the single cell that is the start of a new life (known as sex cells)
• Male gamete is smaller than the female one – due to this male gametes can move whereas the female gamete moves less or not at all
• Sperm has a smaller volume than the egg cell – using the tail to swim to the egg
• Genes are passed through to offspring by gametes
• Gametes contain one chromosome of each type of haploid
• Nucleus of a gamete only has one allele of each gene
• Female/male parents make an equal genetic contribution to their offspring

3.4.U3 ​The two alleles of each gene separate into different haploid daughter nuclei during meiosis.​  

• State the outcome of allele segregation during meiosis.

• During meiosis a diploid nuclei in a germ cell divides to produce 4 haploid  nuclei
• If an individual has two of the same allele AA for a particular gene, all 4 haploid cells will contain the allele A. This is the same if the alleles for the gene are aa
• If an individual has two different alleles for a particular gene such as Aa, the haploid gametes will contain 50% A and 50% a for that specific gene
• The separation of the alleles into different nuclei is called segregation

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

• When the gametes (n) fuse to form a zygote (2n), two copies of each gene exist in the diploid zygote
• The zygote may contain two of the same allele AA or aa or two different alleles such as Aa

For any given gene, the combination of alleles can be categorised as follows:

• If the maternal and paternal alleles are the same, the offspring is said to be homozygous for that gene
• If the maternal and paternal alleles are different, the offspring is said to be heterozygous for that gene
• Males only have one allele for each gene located on a sex chromosome and are said to be hemizygous for that gene

3.4.U5​ Dominant alleles mask the effects of recessive alleles but co-dominant alleles have joint effects. 

• Define dominant allele and recessive 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
• Define codominant alleles.
• Using the correct notation, outline an example of codominant alleles

3.4 U6 ​Many genetic diseases in humans are due to recessive alleles of autosomal genes, although some genetic diseases are due to dominant or co-dominant alleles. 

•  Define “carrier” as related to genetic diseases.
• Explain why genetic diseases usually appear unexpectedly in a population

Many genetic diseases are caused by recessive alleles contained on the autosomal chromosomes (chromosome 1-22). The disease would only be expressed if an individual has two recessive alleles (i.e. aa). If an individual has one of the dominant alleles (i.e. Aa), they will not show symptoms of the disease. These people are known as carriers. They can pass this allele on to their offspring.

If the other parent is also a carrier then their offspring have a 25% chance of getting the disease. A small number of diseases are co-dominant, such as sickle cell anemia which was studied in 3.1. HAHA – do not have sickle cell anemia, HAHS – mild anemia, HSHS – severe anemia. An example of a recessive genetic disease is cystic fibrosis and a dominant disease is Huntington’s Disease

3.4.U7 ​Some genetic diseases are sex-linked. The pattern of inheritance is different with sex-linked genes due to their location on sex chromosomes. 

• Describe why it is not possible to be a carrier of a disease caused by a dominant allele.
• Outline inheritance patterns of genetic diseases caused by dominant alleles
•  Define sex linkage

• These are patterns of inheritance where the ratios are different in males and females because the gene is located on the sex chromosomes
• Generally, sex-linked diseases are on the X chromosome
• Sex-linked inheritance for eye colour is observed and studied in Drosophila (fruit flies)
• Explain sickle cell anemia as an example of a genetic disease caused by codominant alleles.

3.4.U8 ​Many genetic diseases have been identified in humans but most are very rare.

• Outline Thomas Morgan’s elucidation of sex linked genes with Drosophila.
• Use correct notation for sex linked genes.
• Describe the pattern of inheritance for sex linked genes.
• Construct Punnett grids for sex linked crosses to predict the offspring genotype and phenotype ratios.​

• There are over 6000 identified genetic disorders, most of these diseases are caused by rare recessive alleles that follow Mendelian genetics
• Even though this might seem like a lot, most of the human population does not suffer from a genetic disorder and since you need both recessive alleles, these diseases are very rare

3.4.U9 Radiation and mutagenic chemicals increase the mutation rate and can cause genetic diseases and cancer

• List five example genetic diseases.
• Explain why most genetic diseases are rare in a population.

​This is in general why you don’t expose yourself to gamma rays and dangerous chemicals.

• new alleles are formed form other alleles by gene mutation
• gene mutation is a random change to the base sequence of a gene
• Mutation can increase through enough energy to cause chemical changes in DNA // some chemical substances cause chemical changes

3.4.A1 ​Inheritance of ABO blood groups  

• Describe ABO blood groups as an example of complete dominance and codominance.
• Outline the differences in glycoproteins present in people with different blood types

• Human blood types are an example of both multiple alleles (A, B, O) and co-dominance (A and B are co-dominant).
• Co-dominant alleles such as A and B are written as a superscript (IA and IB). The (I) represents immunoglobulin. Blood type O is represented by (i).
• Both IA and IB are dominant over the allele (i).
• A, B and O alleles all produce a basic antigen (glycoprotein) on the surface of the red blood cells
• The allele for blood type B alters the basic antigen by adding a galactoseto the glycoprotein. Individuals that do not have this allele and are exposed to blood type B, will produce Anti-B antibodies that will attack and destroy these red blood cells (RBC)
• The allele for blood type A alters the basic antigen by adding an acetylgalactosamine. So individuals that do not have the A allele will produce Anti-A antibodies that will attack and destroy these RBC’s
• The allele for blood type O produces the basic antigen that will be present on the cell membrane of these RBC’s. Individuals with blood type O will produce both Anti-A and Anti B antibodies if exposed to either A or B blood cells
• Individuals that have both A and B alleles will have both of the antigen modifications. Hence, the alleles for A and B are co-dominant. If exposed to blood type A or B, no Anti-A or Anti-B antibodies will be produced.
• If individuals with blood type A, B or AB are exposed to blood type O, no immune response will occur because blood type O only contains the basic antigen

• ​ Describe the cause and effect of red-green color blindness.
• Explain inheritance patterns of red-green color blindness.
• Describe the cause and effect of hemophilia.
• Explain inheritance patterns of hemophilia

• Color blindness and hemophilia are both examples of sex linkage.
• Color blindness and hemophilia are produced by a recessive sex-linked allele on the X chromosome.
• X-linked recessive diseases such as color blindness and hemophilia are more common in males because males only carry one X chromosome, therefore if they inherit the X chromosome with the disease, they will have the disease.
• On the other hand, since females have two X chromosomes, if they inherit one X chromosome with the disease; they have another normal X chromosome to make the correct gene product. These individuals are considered carriers.
• Since male offspring have to receive a Y from their father, they will always inherit the colorblind or hemophilia allele from their mother; not the father.
• Males that have the disease can only pass the colorblind or hemophilia allele onto their daughters. Their sons will receive the Y chromosome.
• Females can only get X-linked recessive diseases if the mother happens to be a carrier of the disease (or has the disease) and the father also has the disease.
• Therefore sex-linked diseases are rare in 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.
• Outline the inheritance pattern of cystic fibrosis.
• Outline the inheritance pattern of Huntington’s disease.
• List effects of Huntington’s disease on an affected individual

Cystic Fibrosis

Cystic fibrosis is a autosomal recessive disease caused by an allele of the CFTR gene on chromosome 7
The CFTR gene codes for a chloride ion channel protein that transports chloride ions into and out of cells. Chloride is a component of sodium chloride, a common salt found in sweat. Chloride also has important functions in cells; for example, the flow of chloride ions helps control the movement of water in tissues, which is necessary for the production of thin, freely flowing mucus.
Mutations in the CFTR gene disrupt the function of the chloride channels, preventing them from regulating the flow of chloride ions and water across cell membranes. As a result, cells that line the passageways of the lungs, pancreas, and other organs produce mucus that is unusually thick and sticky. This mucus clogs the airways and various ducts, causing the characteristic signs and symptoms of cystic fibrosis

• Humans have two copies of the Huntingtin gene (HTT), which codes for the protein Huntingtin (Htt)
• Huntington’s disease is dominantly inherited. Meaning only one bad copy of the gene from either the mother or father will result in Huntington’s disease.
• Therefore, children of people affected with the disease have a 50% chance of getting that allele from an affected parent.
• If both parents have Huntington’s disease, offspring have a 75% chance of being affected by the disease.
• Huntington’s disease is a neurodegenerative genetic disorder that affects muscle coordination and leads to mental decline and behavioral symptoms
• In Huntington’s disease, a repetition of a CAG sequence in the gene encoding for the protein Huntingtin makes it clump together in our brain cells, ultimately making the brain cell die.
• The exact mechanism of the disease is still being researched; however, this is what is current research suggests.
• The repetitive glutamates (CAG) in the Huntington protein change the shape of the brain cells, affecting their function. The glutamate sends signals that constantly over-excite brain cells. Their overexcitement leads to cell damage, and ultimately cell death.

The radioactive isotopes released from the blasts at Hiroshima and Nagasaki and the nuclear plant failure at Chernobyl released radioactive isotopes into the air. After the bombing, 17,500 of the survivors developed tumors but only 850 of those could be attributed to the radioactive isotopes released from the bomb. As for mutations in children, there has been no evidence found but there were probably some (they were just not statistically significant. The general idea is that the bomb did not have the radioactive fallout most believe it to have had

Chernobyl, on the other hand, had far-reaching environmental effects. Pine trees in a 4 square kilometer area from the reactor turned brown and died. Horses and cattle near the plant died due to damage in their thyroid glands, there was an accumulation of radioactive caesium in fish as far away as Germany. 6000 cases of thyroid cancer have been linked to the accident, however, there is no clearly demonstrated increase in leukemia and other “solid cancers. However, animal life has started to thrive in the area because humans can’t live there.

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

• Define monohybrid, true breeding, hybrid, F1 and F2.
• Determine possible alleles present in gametes given parent genotypes.
• Construct Punnett grids for single gene crosses to predict the offspring genotype and phenotype ratios

• Monohybrid inheritance is the inheritance of a single gene.
• The trait coded for by the gene is controlled by different forms of the gene called alleles.
• A Punnett square or grid is a tool used to solve genetic problems.
• For example:
• Mendel studied many different traits related to pea plants.
• One example is seed color. In pea plants, yellow seeds are dominant over green peas.
• If a pea plant that is heterozygous for yellow peas is crossed with a plant with green peas, what are the genotypes and phenotypes of the first generation (F1) of pea plants?

3.4.S2 ​Comparison of predicted and actual outcomes of genetic crosses using real data.  

• Explain the reason why the outcomes of genetic crosses do not usually correspond exactly with the predicted outcomes.
• Describe the role of statistical tests in deciding whether an actual result is a close fit to a predicted result.

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

• Outline the conventions for constructing pedigree charts.
• Deduce inheritance patterns given a pedigree chart

The pedigree above most likely displays an x-linked recessive disease because the mother is a carrier, and she gives the allele to her two sons but not her daughter. The daughter could have inherited the allele carrying the trait being studied. However, one can see in the 3rd generation, the boy does not have the trait. Since he does not have the trait, she probably did not inherit the affected allele. You would have to look to the next generation to get a better idea if the daughter was a carrier or not.

For co-dominance, the main letter should relate to the gene and the suffix to the allele; both upper case. For example, Roan horses that are co-dominant could be represented as RB and Rw, respectively. For sickle-cell anemia, HbA is normal and Hbs is sickle cell.