Mendel’s Genetics

Course: B.Tech BiotechSubject: Basic of Life Science

Unit: IV

Mendel’s Work

• Mendel experimented with thousands of Pea plants looking at their different traits to understand the process of heredity. His discoveries form the foundation of genetics.

• Heredity –the passing of physical characteristics from parents to

offspring• Trait- each different form of a characteristic• Genetics – the study of heredity

1.

Mendel’s experiments

2.

Mendel’s laws of Inheritance

• Gregor Mendel : conducted hybridisation experiments on garden peasfor seven years (1856-1863) and proposed the laws of inheritance inliving organisms.

• Mendel investigated characters in the garden pea plant that were manifested as two opposite traits, e.g., tall or dwarf plants, yellow or

green seeds.

• This allowed him to set up a basic framework of rules governing inheritance, which was expanded on by later scientists to account for all the diverse natural observations and the complexity inherent in them.

• Mendel : artificial pollination/cross pollination experiments using several true-breeding pea lines.

• A truebreeding line is one that, having undergone continuous self-pollination, shows the stable trait inheritance and expression for several generations.

• Mendel selected 14 true-breeding pea plant varieties, as pairs which were similar except for one character with contrasting traits.

• Some of the contrasting traits selected were smooth or wrinkled seeds, yellow or green seeds, smooth or inflated pods, green or yellow pods and tall or dwarf plants

Mendel’s Experiments

Flowering Plant anatomyPistil – produces the female

sex cells or eggs

Stamens – produce pollen which contains the male sex cells or sperm

Fertilization is when the egg and sperm join forming a new organism

In plants the pollen must reach the pistil for fertilization to occur. This is called pollination

3.

Pollination• Pea plants usually self-

pollinate. The pollen from their stamens lands of the same plants pistils.

• Mendel developed a method to cross-pollinate

pea plants. He took pollen from one pea plant and brushed it onto the pistil of another.

4.

Crossing Pea Plants

• Mendel crossed plants with contrasting traits

• Ex. Tall plants with short plants• Started with pure bred plants –

a purebred organism is one who is the offspring of many

generations of that have the same trait

5.

The F1 Offspring

• Mendel crossed purebred tall with purebred short Parental (P) generation

• Tall x short• Offspring from the cross are called F1 (filial)

• All F1 offspring were tall

6.

7.

8.

F2 offspring

• When F1 were full grown, Mendel allowed them to self-pollinate

• F2 were a mix of tall and short

• ¾ were tall and ¼ were short

9.

Experiments with OtherTraits

• Mendel crossed pea plants with other contrasting traits such as seed shape, seed color, seed coat color, etc.

• In all crosses the F1 generation had only 1 form of the trait• In the F2 generation the “lost” form reappeared in ¼ of the

plants.

10.

Dominant and Recessive Alleles

• Mendel’s Conclusion • factors control the inheritance

of traits in peas. • They exist in pairs • The female parent contributes

one factor and the male parent contributes the other factor

• One factor in a pair can mask or hide the other factor

11.

Law of segregation

12.

Inheritance of two gene:

• Crossed pea plants that differed in two characters, as is seen in the cross between a pea plant that has seeds with yellow color and round shape and one that had seedsof green color and wrinkled shape.

• Mendel found that the seeds resulting from the crossing of the parents, had yellow colored and round shaped seeds.

Inheritance of two gene:

13.

14.

Law of Independent Assortment

• In the dihybrid cross, the phenotypes round, yellow; wrinkled, yellow; round, green and wrinkled, green appeared in the ratio 9:3:3:1.

• Such a ratio was observed for several pairs of characters that Mendel studied.

• The ratio of 9:3:3:1 can be derived as a combination series of 3 yellow: 1 green, with 3 round : 1 wrinkled.

• This derivation can be written as:• (3 Round : 1 Wrinkled) (3 Yellow : 1 Green) = 9 Round,

Yellow : 3 Wrinkled, Yellow: 3 Round, Green : 1 Wrinkled, Green

each gamete having either R or r, it should also have the allele Y or y.

The important thing to remember here is that segregation of 50 per cent R and 50 per cent r is independent from the segregation of 50 per cent Y and 50 per cent y.

50 per cent of the r bearing gamete has Y and the other 50 per cent has y.

Similarly, 50 per cent of the R bearing gamete has Y and the other 50 per cent has y.

Thus there are four genotypes of gametes (four types of pollen and four types of eggs).

The four types are RY, Ry, rY and ry each with a frequency of 25 percent or ¼th of the total gametes produced.

Sex determination

Henking (1891) could identified a specific nuclear structure throughout spermatogenesis in a few insectsObservation: 50 per cent of the sperm received this structure

after spermatogenesis, whereas the other 50 per cent sperm did not receive it.

this structure - X body , but he could not explain its significance. investigations by other scientists : ‘X body’ X body : chromosome X-chromosome. large number of insects the mechanism of sex determination: XO type, i.e., all eggs bear an additional X-chromosome besides the otherchromosomes (autosomes).

• some of the sperms : X-chromosome whereas some do not.• Eggs fertilised by sperm having an X-chromosome

become females and, those fertilised by sperms that donot have an X-chromosome become males.

• Among the males an X-chromosome is present but its counter part is distinctly smaller and called the Y-chromosome.

• the males have autosomes plus XY, while female haveautosomes plus XX.

• The sex of a child will be determined by types of sperm.• As the numbers of X bearing sperm is equal to the numbers

of Y bearing sperm, the chance of having baby boy and baby girl for each birth is 50%.

• Ratios of males to females is 1:1

Both in humans andin Drosophila, the female has a pair ofXX chromosomes (homogametic) and themale XY (heterogametic) composition;

15.

16.

Mutation

• Mutation : alteration of DNA sequences.• changes in the genotype and the phenotype of

an organism. • In addition to recombination, mutation is

another phenomenon that leads to variation in DNA.• one DNA helix runs continuously from one end to

the other in each chromatid, in a highly supercoiled form.

• Loss or gain of a segment of DNA : alteration in chromosomes.

• Since genes are known to be located on chromosomes, alteration in chromosomes results in abnormalities or aberrations.

• Chromosomal aberrations : cancer cells.

Types of Mutation

1.Point Mutation : change in a single base pair of DNA.

• E.g., sickle cell anemia2. Frame shift Mutation : Deletions and insertions

of base pairs of DNA.3. Mutagen :chemical and physical factors that

induce mutations.• UV radiations can cause mutations in organisms –

it is a mutagen.

Genetic disorder• 1. Pedigree Analysis:• In the pedigree analysis the inheritance of a particular trait is

represented in the family tree over generations.

• In human genetics, pedigree study provides a strong tool, which is utilized to trace the inheritance of a specific trait, abnormality or disease.

• Some of the important standard symbols used in the pedigree analysis.

• A number of disorders in human beings have been found to be associated with the inheritance of changed or altered genes or chromosomes.

17.

• 2. Mendelian Disorder:• mutations in single genes.• Transmitted to offspring.• Mendelian disorders can be traced in a family by

the pedigree analysis.• Examples: Hemophilia, Cystic fibrosis, Sickle-cell

anaemia, Color blindness, Phenylketonuria, Thalesemia, etc.

Hemophilia, the royal disease• Hemophilia is the oldest

known hereditary bleeding disorder.

• Caused by a recessive gene on the X chromosome.

• There are about 20,000 hemophilia patients in the United States.

• One can bleed to death with small cuts.

• The severity of hemophilia is related to the amount of the clotting factor in the blood. About 70% of hemophilia patients have less than one percent of the normal amount and, thus, have severe hemophilia.

• Hemophilia is a group of hereditary genetic disorders that impair the body's ability to control blood clotting or coagulation, which is used to stop bleeding when a blood vessel is broken.

• Haemophilia A (clotting factor VIII deficiency) is the most common form of the disorder, present in about 1 in 5,000–10,000 male births.

• Haemophilia B (factor IX deficiency) occurs in around 1 in about 20,000–34,000 male births.

• Like most recessive sex-linked, X chromosome disorders, haemophilia is more likely to occur in males than females. This is because females have two X chromosomes while males have only one, so the defective gene is guaranteed to manifest in any male who carries it. Because females have two X chromosomes and haemophilia is rare, the chance of a female having two defective copies of the gene is very remote, so females are almost exclusively asymptomatic carriers of the disorder.

• Female carriers can inherit the defective gene from either their mother or father, or it may be a new mutation.

• Haemophilia lowers blood plasma clotting factor levels of the coagulation factors needed for a normal clotting process.

• Thus when a blood vessel is injured, a temporary scab does form, but the missing coagulation factors prevent fibrin formation, which is necessary to maintain the blood clot.

• A haemophiliac does not bleed more intensely than a person without it, but can bleed for a much longer time.

• In severe haemophiliacs even a minor injury can result in blood loss lasting days or weeks, or even never healing completely.

• In areas such as the brain or inside joints, this can be fatal or permanently debilitating.

Phenylketonuria or PKU

People with PKU cannot consume any product that contains aspartame.

PKU is a metabolic disorder that results when the PKU gene is inherited from both parents

Caused by a deficiency of an enzyme which is necessary for proper metabolism of an amino acid called phenylalanine.

PKU• Phenylalanine is an essential amino acid

and is found in nearly all foods which contain protein, dairy products, nuts, beans, tofu… etc.

• A low protein diet must be followed. • Brain damage can result if the diet is not

followed causing mental retardation…and mousey body odor.

• Phenylketonuria (PKU) is an autosomal recessive metabolic genetic disorder characterized by homozygous or compound heterozygous mutations in the gene for the hepatic enzyme phenylalanine hydroxylase (PAH), rendering it nonfunctional. This enzyme is necessary to metabolize the amino acid phenylalanine (Phe) to the amino acid tyrosine (Tyr).

• When PAH activity is reduced, phenylalanine accumulates and is converted into phenylpyruvate (also known as phenylketone), which can be detected in the urine.

• Untreated PKU can lead to intellectual disability, and other serious medical problems. The mainstream treatment for classic PKU patients is a strict PHE-restricted diet supplemented by a medical formula containing amino acids and other nutrients. In the United States, the current recommendation is that the PKU diet should be maintained for life.

PKU

Sickle Cell Anemia

• An inherited, chronic disease in which the red blood cells, normally disc-shaped, become crescent shaped. As a result, they function abnormally and cause small blood clots. These clots give rise to recurrent painful episodes called "sickle cell pain crises".

18.

Sickle Cell

• Sickle cell disease is most commonly found in African American populations. This disease was discovered over 80 years ago, but has not been given the attention it deserves.

• 3. Chromosomal disorders• caused due to absence or excess or abnormal arrangement of one

or more chromosomes.• Failure of segregation of chromatid during cell division cycle results

in the gain or loss of a chromosome(s), called aneuploidy.For example,• Down’s syndrome results in the gain of extra copy of chromosome

21.

• Failure of cytokinesis after telophase stage of cell division results in an increase in a whole set of chromosomes in an organism and, this phenomenon is known as polyploidy.

Down’s Syndrome

• Caused by non-disjunction of the 21st chromosome.

• This means that the individual has a trisomy (3 – 2lst chromosomes).

• Down syndrome (DS) or Down's syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. It is typically associated with physical growth delays, characteristic facial features, and mild to moderate intellectual disability.

• Down syndrome is the most common chromosome abnormality in humans, occurring in about one per 1000 babies born each year.

Down’s Syndromeor Trisomy 21

19.

Symptoms of Down Syndrome

• Upward slant to eyes.• Small, flattened nose.• Small mouth, making tongue appear large.• Short neck.• Small hands with short fingers.• Low muscle tone.• Small skin folds at the inner corners of the eyes.• Excessive space between first and second toe and • Mental retardation

Kleinfelter’s syndrome(or Klinefleter’s)

• Disorder occurring due to nondisjunction of the X chromosome.

• The Sperm containing both X and Y combines with an egg containing the X, results in a male child.

• The egg may contribute the extra X chromosome.

XXY

• Males with some development of breast tissue normally seen in females.

• Little body hair is present, and such person are typically tall, have small testes.

• Infertility results from absent sperm.• Evidence of mental retardation may or may not

be present.

Web and Books References

• Book name : 12th std NCERT • Book name : Genome by T.A.Brown

• http://www.ndsu.edu/pubweb/~mcclean/plsc431/mutation/mutation4.htm

• http://en.wikipedia.org/wiki/Mutation• http://en.wikipedia.org/wiki/Mendelian_inheritance• http://www.hobart.k12.in.us/jkousen/Biology/mendel.htm

Management 8/e - Chapter 8 53

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1/10/10/153/375/899/190/21.05.08-shivangi-biology-10.2.9.1.2_SG_html_mdb3db29.png

• 9. http://www.thisoldearth.net/Images/Mendal_peas.jpg• 10.http://wallpapersfun.files.wordpress.com/2011/07/gregormendelpeas12.jpg

• 11. https://online.science.psu.edu/sites/default/files/biol011/Fig-6-2-F2-Generation.jpg

• 12. http://www.prism.gatech.edu/~gh19/b1510/segregat.gif• 13.http://biotechlearn.org.nz/var/biotechlearn/storage/images/themes/

mendel_and_inheritance/images/inheritance_of_multiple_traits_in_peas/457375-1-eng-AU/inheritance_of_multiple_traits_in_peas_oversize.jpg

• 14.http://activity.ntsec.gov.tw/lifeworld/english/content/images/en_gene_c4d.jpg

• 15. & 16. & 17. 12th NCERT Book• 18.http://thumb1.shutterstock.com/display_pic_with_logo/

1584560/135245900/stock-photo-red-blood-cells-blood-elements-red-blood-cells-responsible-for-oxygen-carrying-over-regulation-135245900.jpg

• 19.http://592f46.medialib.glogster.com/media/d868c55770ddebb16f361e00d66bf4cadca273f7672eb2e7c745a44ca3c2010e/trisomy.jpg

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