Theoretical Genetics

4.3

4.3.1 Define genotype, phenotype, dominant allele, recessive allele, recessive allele, codominant alleles, locus, homozygous,

heterozygous, carrier, and test cross

• Genotype: • Genotype: The symbolic representation of a pair of alleles

possessed by an organism, typically represented by two letters.

• The genetic constitution of an organism• Examples: TT, Tt, tt.Phenotype• Phenotype: The characteristics or trait of an organism- the

way in which the genotype is expressed (appearance of an organism)

• Examples: five fingers on each hand, color blindness.

Definitions

• Dominant allele• An allele that has the same effect on the phenotype whether

it is paired with the same allele or a different one. Dominant alleles are always expressed in the phenotype.

• An allele that has the same effect on the phenotype in a heterozygous individual (where it is combined with a recessive allele) as in a homozygous individual (where there are two copies of the dominant allele)

• Recessive allele: an allele that has an effect on the phenotype in homozygous individuals (where there are two copies of the recessive allele). In heterozygous individuals the recessive allele is hidden by the dominant allele.

Definitions

• Codominant alleles: pairs of alleles that both affect the phenotype when present in a heterozygote.

• Locus: the particular position on homologous chromosome of a gene. Each gene is found at a specific place on a specific pair of chromosome. So alleles of the same gene occupy the same locus.

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• Homozygous: having two identical alleles of a gene.• Examples: TT is a genotype which is homozygous

dominant whereas tt is the genotype of someone who is homozygous recessive for that trait.

• Heterozygous: Having two different alleles of a gene.• Example: Tt is a heterozygous genotype.• Test cross: testing a suspected heterozygote by

crossing it with a known homozygous recessive.

carrier

• Carrier: an individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this allele.

• An individual with a recessive allele of a gene that does not affect their phenotype –especially one responsible for a genetic disorder such as color blindness or hemophilia.

4.3.2 Determine the genotypes and phenotypes of the offspring of a monohybrid cross using a Punnet grid.

Monohybrid Cross• We can study one of the characteristics Gregor Mendel used in his experiments. He studied

the size of pea plants and found that ‘tall’ is dominant over ‘short’. If we start the experiment with 2 pure breeding (homozygous) plants of contrasting traits (tall and short), we will obtain an F1 (First Filial generation) which has the dominant phenotype (tall) but is heterozygous.

• When self-fertilizing the F1, we will obtain an F2 (Second filial generation) which will appear ¾ dominant (tall) and ¼ recessive (short)

• Start by writing the phenotypes for the relevant gene and what the corresponding genotypes are as in the following table.

• Possible phenotypes• Corresponding genotypes• Tall• TT or Tt• Short• Tt• Then write out the cross. Make sure to include both genotypes and phenptypes of the

parents (P), as well as the genotype and phenptype of the offsproing (F1).

Monohybrid Cross

http://www.jdenuno.com/PDFfiles/Monohybrid.pdfYou can go to this site and have practice for Monohybrid cross using Punnet grid.

Drag and drop Genetics Monohybrid

• http://www.zerobio.com/drag_gr11/mono.htm

• Practice Monohybrid cross by using this link

4.3.3 State that some genes have more than two alleles (multiple alleles)

• We have considered till now two possibilities for a gene: dominant or recessive. With two alleles, three different genotypes are possible which produce two different phenotypes.

• Sometimes there are three or more alleles for the same gene. It is then known as multiple alleles. A good example of this the ABO blood type in humans.

• IA, IB, and i

4.3.4 Describe ABO blood groups as an example of codominance and multiple alleles

Blood Group

Inheritance of Blood Group

• The principle of inheritance discovered by Mendel in pea plants also operate in other plants and in animals. There are however, sometimes differences and these are demonstrated by the inheritance of ABO blood groups in humans –codominance and multiple alleles.

• Work out a cross involving multiple alleles.• Cross between a person having blood group A

(IAi) and blood group B ( IBi)

Inheritance of Blood group

The above punnet square shows how a female with a blood group B and a male with a blood group A can have four children, each with different blood group. Both parents would have to be heterozygous in order to produce a type O child.

Inheritance of Blood group (page number 24 Biology Study Guide by Andrew Allott

CO-DOMINANCE

• Not all genes have dominant and recessive alleles. Some have alleles that are both expressed together in the heterozygote individuals.

• Co-dominant alleles have three phenotypes, one for each genotype.• In humans there are a number of conditions that are co-dominant. • Example

Sickle cell anemia is a genetic disease which affects the hemoglobin of the red blood cells. Hemoglobin is normally a ball-shaped molecule but the sickle cell allele makes it form long strands. The red blood cell carrying these molecules distorts into characteristic long shapes.

• The shape of the hemoglobin molecule is controlled by two alleles:• Normal Hemoglobin allele

Sickle Cell Hemoglobin allele

• There are three phenotypes• Normal

Normal individuals have two normal hemoglobin alleles• Sickle cell anemia, a severe form where all the red blood cells are affected. Sickle

cell anemia patients have two sickle cell alleles in their genotype• Sickle cell trait, a mild condition where 50% of the red blood cells are affected.

Sickle cell trait individuals are heterozygotes, having one of each allele.• Symbols for codominant alleles

Because both alleles are expressed in the heterozygote they are considered codominant and both take a capital case letter. An index letter identifies the allele.

• Therefore:• Normal haemoglobin allele is HbN

• Sickle cell allele is HbS

Sickle Cell Anaemia

Unusual proportionsBecause the heterozygotes have their own phenotype this gives rise to different proportions amongst their offspring compared with crosses between heterozygotes for dominant and recessive alleles.

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4..3.5 Explain how the sex chromosomes control gender by referring to the inheritance of X and Y chromosomes in humans

• SEX CHROMOSOMES AND GENDER• Two chromosomes determine the gender of a child (whether it is male or

female). These are called sex chromosomes.• The X chromosome is relatively large and carries more genes.• The Y chromosome is much smaller and carries only a few genes.• If two X chromosomes are present in human embryo and no Y chromosome,

it develops into a girl.• If one X chromosome and one Y chromosome are present, a human embryo

develops into a boy.• When women reproduce (produce gametes) they pass on one X

chromosome in the egg.• When men reproduce (produce sperm) they pass on either one X or one Y

chromosome in the sperm, so the gender of the child depends on whether the sperm that fertilizes the egg is carrying X or a Y chromosome.

Inheritance of gender in human

The 23rd pair of chromosomes is called the sex chromosome because they determine if a person is a male or a female. The X chromosome is longer than the Y

chromosome.

•The 23rd pair is the only pair in which it is possible to find two chromosomes that are very different in size and shape.•In human females there are two x chromosomes. When women produce gametes each egg contains one X chromosome. Human males have one X chromosome and one Y chromosome. When males produce sperm cells, half of them contain one X chromosome and half contain one Y chromosome. •As a result when an egg cell meets a sperm cell during fertilization, there is always a 50% chance that the child will be a boy and a 50% chance that the child will be a girl.•XX = female•XY = male.

Problems based on the concepts learnt

• http://www.eastsideibbiology.com/genetics%20wkst.pdf

• Work out the problems given in the above link

4.3.6 State that some genes are present on the X chromosome and absent from the shorter Y chromosome in in humans

• GENES CARRIED ON THE SEX CHROMOSOME• Because the Y chromosome is significantly smaller than the X

chromosome, it has fewer loci and therefore fewer genes than the X chromosome. This means that in males, the majority of alleles present on the X chromosome have no alleles to pair with on the smaller Y chromosome.

• The X chromosome is relatively larger and several genes located on the X chromosome such as the ability to see colors and hemophilia are absent on the Y chromosome. These genes are said to be ‘X-linked’.

• This means that a male with one allele for color blindness on the X chromosome, will be color blind since there is no locus on the Y chromosome.

4.3.7 Define sex linkage

• Sex linkage is the association of a characteristic with gender, because the gene controlling the characteristic is located on a sex chromosome.

• If a gene is carried on the X chromosome, the pattern of inheritance is different for males and female there is sex linkage.

• Sex-linked genes are almost always located on the X chromosome. Females have two X chromosomes and therefore have two copies of sex linked genes. Males have only one X chromosome and therefore only have one copy of sex linked genes.

Genetic Disorder

4.3.8Describe the inheritance of color blindness and hemophilia as examples of sex linkage

• Hemophilia and red-green color-blindness are examples of sex-linked characteristics.

• Hemophilia is a disorder in which blood does not clot properly. People with hemophilia risk bleeding to death from what most people would consider a minor injury such as a bruise, which would rupture many tiny blood vessels. Such bleeding can occur in internal organs. Medical treatment such as injections help to give people affected by hemophilia a better quality of life.

Color Blindness• Color blindness is the inability to distinguish between certain colors,

often green and red. To people who are color blind, the two colors look the same.

• Color blindness is a condition that can be caused by genetic factors. Human eyes contain cells with different pigments that absorb different wavelengths (colors) of light. If this pigment absorbs light, a message is sent to the brain and we see a color. The ability to make the pigment is a dominant allele, the recessive allele will not allow the pigment to be made.

ALLELES AND GENOTYPES OF SEX-LINKED TRAITS

• Since the alleles for both color blindness and hemophilia are found only on the X chromosome, the letter X is used in representing them.

• Alleles on the X chromosome for color blindness• XB = allele for the ability to distinguish colors;• Xb = recessive allele for color blindness.• Alleles on the X chromosome for hemophilia• XH = allele for the ability to clot blood.• Xh = allele for hemophilia.• In both cases, there is no allele on the Y chromosome, so Y is

written alone without any subscript. Here are all the possible genotypes for color blindness.

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• XBXB gives the phenotype of a non-affected female;

• XBXb gives the phenotype of a non-affected female who is a carrier,

• XbXb gives the phenotype of an affected female;• XBY gives the phenotype of a non-affected male;• XbY gives the phenotype of an affected male.• In the above list B and b can be replaced by H

and h to show the genotypes for hemophilia.

• Sex-linked alleles such as Xb are rare in most populations of humans worldwide. For this reason it is unlikely to get one and much less probable to get two. This is why so few women are color blind: their second copy of the gene is likely to be the dominant allele for full-color vision and mask the recessive allele.

• Only women can be heterozygous XBXb , and as a result they are the only ones who can be carriers.

• Since men do not have second X chromosome, there are only two possible genotypes XBY or XbY in relation to color blindness. With just the one recessive allele, a man will be color blind. Usually people need two recessive alleles to express the trait, and with one they are carriers.

• Men cannot be carriers for X-limked alleles.• http://kvhs.nbed.nb.ca/gallant/biology/sex_linkage.html• pedigree chart on colorblindness

Color blindness

Criss cross Inheritance• The transmission of a character from father to grandson through his

daughter is called criss-cross inheritance. It is also called zig-zag inheritance.

In criss-cross inheritance, the character appears in alternate generation only.

The sex linked characters exhibit criss-cross inheritance.

Eg. Haemophilia.

Colour blindness.

Criss Cross Inheritance

Hemophilia

Hemophilia• HEMOPHILIA IS A BLOOD DISORDER. NORMALLY THERE IS A VERY FINE BALANCE

FOR BLOOD CLOTTING. BLOOD SHOULD NOT CLOT WHEN IT IS INSIDE BLOOD VESSELS OR IT WILL BLOCK THE VESSELS BUT IT SHOULD clot when there is injury so that too much blood is lost.

• The process blood clotting involves a number of different proteins, each having their own gene. Even if one these genes has an allele that does not code for the proper protein, the entire process of blood clotting can be disturbed and even very small wounds will not clot.

• In humans the locus for the gene that controls production of a blood clotting factor is on the X chromosome (I.e. and not on the Y chromosome.) this means that if a male has one defective allele he will have the hemophilia condition. However a female will need to have two copies of the defective allele in order to have the condition. Statistically this is much less likely to happen and with the advent of menstruation and child birth, a woman will need blood transfusion containing the clotting factors to survive. Prior to this technique hemophilia was thought to be homozygous lethal in all cases.

Work out the genetics problems and check your answers

• http://www.biology.arizona.edu/mendelian_genetics/problem_sets/sex_linked_inheritance/sex_linked_inheritance.html

• http://images.google.co.in/imgres?imgurl=http://www.execulink.com/~ekimmel/drag_gr11/mono_sq.gif&imgrefurl=http://www.execulink.com/~ekimmel/drag_gr11/sexlink.htm&usg=__uLHo8oH5bFowLpIYTXhINn7Jtdk=&h=206&w=327&sz=2&hl=en&start=28&um=1&tbnid=QmLvNJjAV8BjVM:&tbnh=74&tbnw=118&prev=/images%3Fq%3Da%2Bcross%2Bshowing%2B%2Bhemophilia%26ndsp%3D18%26hl%3Den%26sa%3DN%26start%3D18%26um%3D1

This example shows how two parents, neither of whom have hemophilia, could have a hemophilic son.

• The mother is heterozygous but is not a hemophilic because H is dominant and h is recessive. She is a carrier of the allele for hemophilia. A carrier has a recessive allele of a gene but it does not affect the phenotype because a dominant allele is also present.

• The father is normal and the Y chromosome does not carry either allele of the gene. In the first generation, none of the female offspring are hemophilic because they all inherited the father’s X chromosome which carries the allele for normal blood clotting (H), but there is a 50% chance of a daughter being a carrier.

• There is a 50% chance of a son being hemophilic as half of the eggs produced by the mother carry Xh.

• The chance of a daughter being hemophilic is 0% so the overall chance of offspring being hemophilic is 25%

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4.3.9State that a human female can be homozygous or heterozygous with respect to sex linked genes

• Sex-linked genes are found on the X chromosomes. Since females have two X chromosomes, they can have two dominant alleles ( homozygous dominant), two recessive alleles ( homozygous recessive) or one dominant and one recessive allele ( heterozygous ) . Males have only one X chromosome. This means that the terms homozygous or heterozygous do not apply to males.

4..3.10Explain that female carriers are heterozygous for X-linked recessive alleles

• In humans the locus for the gene that controls the production of a blood clotting factor is on the X chromosome (i.e not on the Y chromosome).

• This means that if a male has one defective allele he will have the hemophilia condition. However a female will need to have 2 copies of the defective allele in order to have the condition. Women can be carriers of a trait. In that case they are heterozygous and will not show the trait but are capable of passing it on.

• Since men have only one X chromosome, the allele on this chromosome will always be expressed. Men have one allele for color blindness and be color blind or have the allele for normal vision and have normal vision. The same applies to hemophilia. Men cannot have the allele without expressing it so they can never be carriers for the X-linked traits.

4.3.11Predict the genotypic and phenotypic ratios of offspring of monohybrid crosses involving any of the above patterns of

inheritance

• Predict means to give an expected result.• 1.The ability to taste the chemical PTC is determined by a single gene in

humans with the ability to taste given by the dominant allele T and inability to taste by the recessive allele t. Suppose two heterozygous tasters (Tt) have a large family.

•a. Predict the proportion of their children who will be tasters and non-tasters. b. Use a Punnett square to illustrate how you make these predictions.is the likelihood that their first child will be a taster? c. What is the likelihood that their fourth child will be a taster?d. what is the likelihood that the first three children of this couple will be non tasters.

• [[Answer]]

Answer to Q. No 1

Work out the problems here and you can verify your answers

• http://click4biology.info/c4b/4/gene4.3.htm#eleven

4.3.12 Deduce the genotypes and phenotypes of individuals in pedigree charts

• Pedigree charts are often used to record blood lines in royal families.

• A pedigree chart shows the members of a family and how they are related to each other .

• The term ‘pedigree’ refers to the record of an organism’s ancestry.

• Pedigree charts are diagrams which are constructed to show biological relationships. In genetics, they are used to show how a trait can pass from one generation to the next. Used in this way for humans, a pedigree chart is similar to a family tree complete with parents, grandparents, aunts, uncles and cousins.

• It helps to predict the likely outcome of the next generation.

USING PEDIGREE CHARTS

• Pedigree charts can be used to deduce whether a character is caused by a dominant or recessive allele and whether it is sex-linked or not. They can also be used to deduce the genotypes of individuals.

• Males are shown as squares and females as circles. If the phenotypes of the members of the family are known, the genotypes can often be deduced.

• http://www.horton.ednet.ns.ca/staff/selig/handouts/bio12/mengenetics/pedigrees.pdf

• (pedigree analysis in genetics)

Pedigree Chart

Huntington’s Disease

• Huntington’s disease( Huntington’s chorea) is caused by a dominant allele which is represented by the letter H. this genetic condition causes severely nerve damage but the symptoms do not show until the person is about 40. As a result, someone who has the gene for this disease does not know if for certain until they have started a career and possibly a family.

• They symptoms are difficulty in walking, speaking, and holding objects. Within a few years, the person loses complex control of his or her muscles and dies an early death.

• Since it is dominant , all it takes is one H in the person’s genetic makeup to cause the condition.

Albinism • The word “albinism” refers to a group of inherited conditions.

People with albinism have little or no pigment in their eyes, skin, or hair. They have inherited altered genes that do not make the usual amounts of a pigment called melanin. One person in 17,000 in the U.S.A. has some type of albinism. Albinism affects people from all races. Most children with albinism are born to parents who have normal hair and eye color for their ethnic backgrounds. Sometimes people do not recognize that they have albinism. A common myth is that people with albinism have red eyes. In fact there are different types of albinism and the amount of pigment in the eyes varies. Although some individuals with albinism have reddish or violet eyes, most have blue eyes. Some have hazel or brown eyes. However, all forms of albinism are associated with vision problems.

Using Test Cross• It is not always possible to discover whether an individual has a gene, or

does not have it, by looking at the individual’s phenotype. If one allele of a gene is dominant and another allele is recessive, an individual with two copies of the dominant allele has the same phenotype as an individual with one dominant and one recessive allele.

• These two genotypes can be distinguished by carrying out a test cross.• In order to find out if the hybrid is homozygous or heterozygous, a test cross

can be conducted.• In a test cross and individual that might be heterozygous is crossed with an

individual that is homozygous recessive.• Example of a test cross.• A farmer is unsure whether his bull is a purebred Hereford or whether it is a

hybrid. If the test cross results give a ratio of 1:1 then, it is a hybrid otherwise it is a homozygous.

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