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Genetics-ll.

posted Aug 15, 2016, 9:45 PM by Upali Salpadoru   [ updated Aug 27, 2016, 11:02 PM ]

Heredity









Prior to the study of this lesson it is essential to be familiar with these terms.

Trait:- This is a characteristic of an individual such as hair colour, or an action such as the ability to roll the tongue.



Genotype :-The functioning and dormant genetic material present in an organism.

Phenotype:- The functioning or visible traits of that organism. The form that is shown.

Alleles:- Different forms of a gene, which may give rise to different phenotypes, (An individual gets two alleles from the two parents.)

Homozygous:-describes two of the same alleles at a specific locus.

Heterozygous:-describes two different alleles at a specific locus


All members in a specie are not the same. There are minor differences. Such variations help the survival of the species.For example if a disease spreads, every individual will not be susceptible to the same extent. Even when a large percentage gets wiped out a few individuals at least will survive and continue the specie. This is how the evolution takes place. The surviving individuals will be slightly different to the original stock.

The traits inherited are of two kinds.

1.Continuous traits.

Characteristics show a wide range of differences.  Eg. Size, skin colour, inteligence.

2. Discrete traits. (Discontinuous)

There are only two possibilities. A rabbit can be either white or black.

Eg. Ability to roll the tongue.


How alleles pass on  traits

The inheritance of a trait is governed by the 2 genes, which are termed alleles. These two come from the two parents. In a pair one allele may be dominant while the other is recessive. If a dominant allele is present it will always exhibit in the phenotype. The recessive allele will only function in the absence of a dominant allele.


Three genotypes are possible with a pair of alleles.

We can illustrate this if we consider the dominant factor brown eyes as ‘R’ and the recessive factor as simple ‘r’ for blue eyes.


Description

Genotype

combination

Phenotype

1.Homozygous dominant.

R + R

Brown eyes.

2. Homozygous recessive.

r  + r.

Blue eyes.

3.Heterozygous

R + r.

Brown eyes.


Gregor Mendel discovered these rules of inheritance long before the discovery of the genes. According to the theory of Mendelian inheritance, variations in phenotype are due to variations in the genotype.

Mendel's Experiment.

He grew two varieties,  of the pea plants, having purple and white flowers,  allowing them to self pollinate so that he could obtain pure bred specimens. Then he systematically recorded the passing of seven characteristics (which we now call traits). Let us select one trait to show what he discovered.
In This case two pure bred varieties are taken and the flowers are cross pollinated. While capital B represents a dominant blue colour , simple w' is for a recessive gene. Combination of the plants shown in the punnet can produce only blue flowers.  This does not mean that the allele to produce white flowers has vanished. It will be there and exhibit it self when the daughter plants are crossed. 
Mendel's Law of Dominance.
.
All BLUE, 'cause that was the dominant in the two pure breds, Look at the Genotype. Recesive gene is there in every one.
After cross pollination of two pure forms having different discrete traits , only one form will appear in the first order of off springs.


Mendel's Law of segregation.
During reduction division to form gametes(Sex cells) two alleles coded for a trait separate.On fertilization they combine. The off springs always bear the ratio 3:1 for dominant:recessive alleles.

Sex determination



Out of the 46 genes of an individual 22 pairs would be identical in males and females. These are called autosomes.  The 23rd pair which is one that determines the sex would be different.  This diagram shows the difference.

In females sex cells are identical, X and X,  while males have two types, X and Y.

Gametes form after reduction division( meiosis) do not have a pair of chromosomes but only one. So the egg cells contain only X chromosomes while a male will have half the number  of  sperms with X and the other half with Y.

X-.chromosome. Has about 1098 genes.

Y chromosome, much smaller has only about 26 genes

X linked chromosomes are also responsible for some abnormalities such as haemophilia and a form of baldness.The Y chromosome has the genes for male characteristics. In the absence of Y, the embryo would produce a female.

As the males carry only one copy of X, they are more liable to get hereditary disorders such as haemophilia and colour blindness..

When  normal cells are stained , only in the case of female cells a stained body becomes visible. This is really the condensed form of an X chromosome. This has become invaluable in some legal cases.

Protein synthesis.

This is the process of making proteins according to the information coded in DNA..  The intermediary for the making of new protein is the RNA.( Ribonucleic acid.)

RNA.jpgFig.      Ribonucleic acid.

RNA varies from DNA from the following:

            1.They are single stranded.

            2. The base cell Thymine is changed to Uracil.

            3.Contains the sugar Ribose instead of deoxyribose.

Three bases in order is sufficient for a particular amino acid.  These are known as triplets or codons.

What are codons or Triplets ?

There are only 20 amino acids in human beings. They are listed here.

code

Amino acid

Possible codons

Order of Triplets

A

Ala

Alanine

GCA, GCC, GCG, GCT

B

Asx

Asparagine or Aspartic acid

AAC, AAT, GAC, GAT

C

Cys

Cysteine

TGC, TGT

D

Asp

Aspartic acid

GAC, GAT

E

Glu

Glutamic acid

GAA, GAG

F

Phe

Phenylalanine

TTC, TTT

G

Gly

Glycine

GGA, GGC, GGG, GGT

H

His

Histidine

CAC, CAT

I

Ile

Isoleucine

ATA, ATC, ATT

K

Lys

Lysine

AAA, AAG

L

Leu

Leucine

CTA, CTC, CTG, CTT, TTA, TTG

M

Met

Methionine

ATG

N

Asn

Asparagine

AAC, AAT

P

Pro

Proline

CCA, CCC, CCG, CCT

Q

Gln

Glutamine

CAA, CAG

R

Arg

Arginine

AGA, AGG, CGA, CGC, CGG, CGT

S

Ser

Serine

AGC, AGT, TCA, TCC, TCG, TCT

T

Thr

Threonine

ACA, ACC, ACG, ACT

V

Val

Valine

GTA, GTC, GTG, GTT

W

Trp

Tryptophan

TGG

X

X

any codon

NNN

Y

Tyr

Tyrosine

TAC, TAT

Z

Glx

Glutamine or Glutamic acid

CAA, CAG, GAA, GAG

*

*

stop codon

TAA, TAG, TGA



Step 1.Transcription.

For the genetic code to be active the two strands have to open up. This happens at different places at the same time catalyzed by an enzyme Helicase. Any one of these strands is used as a template to make a molecule of ribonucleic acid which would be known as mRNA. (‘m’ for messenger)Tr 2.jpg

This copy, called a messenger RNA (mRNA) molecule, leaves the cell nucleus and enters the cytoplasm, where it directs the synthesis of the protein, which it encodes.

This process is assisted by an enzyme called Polymerase .

One end of a DNA polymer contains an exposed hydroxyl group on the deoxyribose; this is known as the 3' end of the molecule. The other end contains an exposed phosphate group; this is the 5' end. So processes such as DNA replication occur in only one direction. All nucleic acid synthesis in a cell occurs in the 5' to 3' direction.
Step 2. Translation

This copy, called a messenger RNA (mRNA) molecule, leaves the cell nucleus and enters the cytoplasm, where it directs the synthesis of the protein, which it encodes.

Translation is the process of translating the sequence of a messenger RNA (mRNA) molecule to a sequence of amino acids during protein synthesis. The genetic code describes the relationship between the sequence of base pairs in a gene and the corresponding amino acid sequence that it encodes. In the cell cytoplasm, the ribosome reads the sequence of the mRNA in groups of three bases to assemble the protein.

A gene can be switched on and off. When a gene is active, the coding and non-coding sequences are copied in a process called transcription, producing an RNA copy of the gene's information. 

The mRNA then leaves the nucleus to the cytoplasm where it is translated via the use of tRNA and ribosomes and amino acids to translate the mRNA strand into a protein. 

When the gene is turned off, a repressor molecule is formed which stops protein synthesis as it covers the promoter site.

There are only 20 Amino acids among all living 

Gregor Mendel  first came across the idea of genes, in the 1860s. 

He studied inheritance in pea plants and hypothesized a factor that conveys traits from parent to offspring. Mendel was also the first to hypothesize independent assortment, the distinction between dominant and recessive traits, the distinction between a heterozygote (A person carrying two different forms of a gene inherited from father and the mother) and homozygote (A person having two similar genes inherited from the two parents), which is the genotype (the genetic material of an organism) and phenotype (the visible traits of that organism).

According to the theory of Mendelian inheritance, variations in phenotype are due to variations in genotype, or the organism's particular set of genes, each of which specifies a particular trait. Different forms of a gene, which may give rise to different phenotypes, are known as alleles. Organisms such as the pea plants Mendel worked on, along with many plants and animals, have two alleles for each trait, one inherited from each parent. Alleles may be dominant or recessive; dominant alleles give rise to their corresponding phenotypes when paired with any other allele for the same trait, whereas recessive alleles give rise to their corresponding phenotype only when paired with another copy of the same allele. But this is not always the case; in this situation the alleles are completely dominant. In other cases the alleles could be incompletely dominant or co-dominant. Incomplete dominance is when a characteristic in the heterozygote s intermediate in the degree between the two homozygotes. And co-dominance is when the heterozygote simultaneously expresses 2 alleles that have different kinds of effect.

 Chromosomes within cells are the carriers of genetic material, and they are made of DNA (deoxyribonucleic acid). DNA consists of a chain made from four types of nucleotide subunits: i.Adenine, ii.Cytosine, iii.Guanine, and iv.Thymine

Each nucleotide sub-unit consists of three components: i. a phosphate group,  ii. deoxyribose sugar ring, and a iii. base. (Adenine and guanine are purines, and cytosine and thymine are pyrimidines.)

 The DNA in a cell has double helix structure, in which two individual DNA strands twist around each other in a right-handed spiral. In this structure, the base pairing rules specify that guanine pairs with cytosine and adenine pairs with thymine (each pair contains one purine and one pyrimidine). 

The base pairing between guanine and cytosine forms three hydrogen bonds, whereas the base pairing between adenine and thymine forms two hydrogen bonds. The two strands in a double helix are complementary.

Q.1.0

Give a single word for these:

1.The functioning and dormant genetic material present in an organism.

2.The functioning or visible traits of that organism. The form that is shown.

3.Different forms of a gene, which may give rise to different phenotypes,

4. Two of the same alleles at a specific locus.

5. Two different alleles at a specific locus.

6. A characteristic showing a wide range of differences.

7. A trait showing only two variations.  

8. The process of making proteins according to the information coded in DNA.

9. The process by which the information in a strand of DNA is copied into a new molecule of messenger RNA (mRNA).

10. Synthesis of amino acids according to a an mRNA sequence.

3x10-30 marks.

Q. 2.0 MCQ.

  1. Two parents have had three female children. What are the chances of getting a male child next time?                              A. 1:3,  B. 3:1,  C.  2:1  D. 1:1.

  2. Which of these can you change?                       A.- Instincts. B.- Inherited traits. C-Acquired knowledge. D.-Inherited behaviors

  3. A rabbit, black (B-dominant) colour, was bred with a white one (b-recessive). There were 3 black off springs and 1 white. What could have been the genotype of the parents.  A.- BB and bb.  B.- Bb and Bb,  C-  bb and BB, D- Cannot be determined.

  4. The flowers of two plants, having white (W -dominant) and blue (b -recessive) were cross pollinated. This resulted in plants only with white flowers. What genotypes could have been possible  for the plant which had white flowers?

         A- WW or Wb,  B-  WW Only.   C-  Wb  only D- Cannot be determined.

  1. Which is not correct for the RNA molecule.                                                           A- It is a double helix. B-It is a single strand.   C- The base cell Thymine is changed to Uracil. D.-Contains the sugar Ribose instead of deoxyribose.

         4x5=20 marks.

Q.3.0

Recognize these cells and answer the questions below. There can be more than one letter to an answer.

cells.jpg

Name the following:-

  1. The somatic cells.     2.  The gamete cells.  3. Sperms     4. Egg cells. 5. A female somatic cell. 6. An abnormal cell.  7. A zygote  8. A fertilized cell that can produce a boy child.

3x8 = 24 maeks.

Q.4.0

Complete these punnets.and comment on the offsprings produced using the table given.

Bd = Black dominant.  Wr = White recessive.


No.

1 example

2

3

4

5

6

7

8

marks

Genotype

B w

7

Phenotype.

black

7

Ratio

Black; White.

F1 Hybrid.

F2 Hybrid.

2

=16 m.


pun.jpg

Q.5.0

Answer these.

  1. What is the molecule that escape from the nucleus to make proteins.

  2. How many amino acids are essential for living organisms?

  3. How many nucleotides are required for codons (triplets) in all amino acids required.

  4. Name the enzyme that helps to split up the DNA molecule.

  5. A molecule that binds to a gene in DNA to stop transcription.

2x5 =10 marks.

For answers click:- Answers-Bio.







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