B.Sc. Cell biology and genetics Paper, Notes for B.Sc Biotechnology students, Coupling and repulsion human genetics, includes examples

1. Linkage and Crossing Over
&
Human Genetics
By
Dr. Krishna
Assistant Professor in Biotechnology
Tumakuru

2. Linkage and Crossing Over:
• A chromosome possesses many genes & all genes present in the chromosome are
inherited together.
• Mendel’s Second Law, or the law of the independent assortment, is valid for genes
located in different chromosomes. These genes segregate independently during meiosis.
• However, Mendel’s Second Law is not valid for phenotypical features conditioned by
genes located in the same chromosome (genes under linkage), since these genes, known
as linked genes, do not separate during meiosis (except for the phenomenon of crossing
over).
• The fruit fly, or drosophila, is suitable for studying genetics because it presents many
distinct traits but only has four chromosomes (one sex chromosome and three
autosomes).
Linkage: Study of inheritance of all genes present in a chromosome together.
• All genes in a chromosome are together referred as linked genes & they form a linkage
group.
• The total number of linkage group in an organism is equal to its haploid number of
chromosomes.

3. Bateson & Punnet:
• While working on sweet pea (Lathyrus odoratus) observed that genes for
flower color & pollen shape remain together and do not assort
independently according to Mendel’s law of Independent assortment.
• Test cross failed to produce 1:1:1:1 ratio & instead produced 7:1:1:7.
• They gave the Coupling & Repulsion theory:
• Coupling: when genes come from the same parent they enter the same
gamete & are inherited together.
• Repulsion: genes are inherited separately when genes come from different
parents & they enter different gametes

5. • T H Morgan’s Linkage: He worked on Drosophila.
• Linked genes show 2 types of arrangement:
• Cis arrangement:
dominant alleles of 2 or more genes are present in one chromosome & its recessive alleles in
its homologue. AB/ab. This is Coupling.
• Trans arrangement: the dominant allele of one pair & recessive of the other pair together
lie in a chromosome. Ab/aB. This is Repulsion.
• Linked genes are of 2 types:
• Complete linkage: genes for 2 or more characters appear together for two or more
generations in their parental combination. They are closely located in the chromosome.
• Incomplete linkage: the parental combination of 2 or more characters are not retained in
the next generations. They are not closely located in the chromosome.
•Linked genes are genes present on the same chromosome.
•Linked genes are inherited together.
•Genes are linearly arranged in a chromosome.
•Strength of linkage: Genes which are closely located show strong linkage & genes which are
located far show weak linkage.
•He stated that Coupling & Repulsion are two aspects of Linkage.

6. Crossing over:
•It is the exchange of segments
between non-sister chromatids of
homologous chromosomes.
•It occurs during pachytene stage
of prophase I in meiosis.
•Crossing over always occurs
between linked genes.
•It produces recombination of
linked genes which play very
important role in evolution.
•Recombination frequency helps
in finding out the distance
between genes. Given by
Sturtevant.
•Recombination frequency helps
in the construction of genetic
maps of the chromosomes.

8. In a dihybrid cross:
• When the test cross result is 1:1, genes are linked and there is no
crossing over.
• If the test cross result is 1:1:1:1 means the genes are independently
assorting (present on separate chromosomes).
• If in the test cross result parental combination is more than 50% &
recombination is less than 50%, the genes are linked and crossing
over has occurred.

9. Coupling and Repulsion Hypothesis of Gene.
Bateson and Punnett in 1906, described a cross in sweat pea, where failure of gene pairs to assort independently
was exhibited. Plants of a sweat pea variety having blue flower (BB) and long pollen (LL) were crossed with those
of another variety having red flower (bb) and round pollen (II). F1 individuals (BbLl) had blue flower and long
pollen.

10. Linkage in Maize: Maize provides a good example of linkage.
Hutchinson crossed a variety of maize having coloured and
full seed (CCSS) with a variety having colourless and
shrunken seeds (ccss). The gene C for colour is dominant
over its colourless allele c and the gene S for full seed is
dominant over its shrunken allele s. All the F1 plants
produced coloured and full seed. But in a test cross, when
such F1 females (heterozygous) are cross pollinated with the
pollen from a plant having colourless and shrunken seeds
(double recessive), four types of seeds are produced (Fig.
5.8).
From the above stated result it is clear that the parental
combinations are more numerous (96.4%) than the new
combination (3.6%). This clearly indicates that the parental
characters are linked together. Their genes are located in the
same chromosome and only in 3.6% individuals these genes
are separated by crossing over. This is an example of
incomplete linkage.

11. Linkage In Drosophila
• Morgan (1911) crossed an ordinary wild type Drosophila with grey body
and long wings (BB VV) with another Drosophila (mutant type) with black
body and vestigial wings (bbvv). All the hybrids in F1 generation are with
grey bodies and long wings (BbVv) i.e., phenotypically like the wild type of
parents. If now a male of F, generation (Bb Vv) is back crossed with a
double recessive female (test cross) having black body and vestigial wings
(bbvv) only parental combinations are formed in F2 generation without the
appearance of any new combinations. The results indicate that grey body
character is inherited together with long wings.
• It implies that these genes are linked together. Similarly, black body
character is associated with vestigial wing. Since only parental
combinations of character appear in the offspring of F2 generation and no
new or non-parental combinations appear, this shows complete linkage.
Complete linkage is seen in Drosophila males.

12. Types of Linkage:
Depending upon the presence or absence of new combinations or non-parental
combinations, linkage can be of two types:
(i) Complete Linkage:
• If two or more characters are inherited together and consistently appear in
two or more generations in their original or parental combinations, it is
called complete linkage. These genes do not produce non-parental
combinations.
• Genes showing complete linkage are closely located in the same
chromosome. Genes for grey body and long wings in male Drosophila show
complete linkage.
(ii) Incomplete Linkage:
• Incomplete linkage is exhibited by those genes which produce some
percentage of non-parental combinations. Such genes are located distantly
on the chromosome. It is due to accidental or occasional breakage of
chromosomal segments during crossing over.

13. • Significance of Linkage:
• (i) Linkage plays an important role in determining the nature of scope
of hybridization and selection programmes.
• (ii) Linkage reduces the chance of recombination of genes and thus
helps to hold parental characteristics together. It thus helps organism
to maintain its parental, racial and other characters. For this reason
plant and animal breeders find it difficult to combine various
characters.

14. Human Genetics
• Autosome:
• Allosome:
Autosomes differ from allosomes because autosomes appear in pairs
whose members have the same form but differ from other pairs in a
diploid cell, whereas members of an allosome pair may differ from
one another and thereby determine sex.
• Human genetics is the study of inheritance as it occurs in human
beings. Human genetics encompasses a variety of overlapping fields
including: classical genetics, cytogenetics, molecular genetics,
biochemical genetics, genomics, population genetics, developmental
genetics, clinical genetics, and genetic counseling.

16. • Autosomal dominant inheritance
• Autosomal traits are associated with a single gene on an autosome (non-sex
chromosome)—they are called "dominant" because a single copy—inherited from either
parent—is enough to cause this trait to appear. This often means that one of the parents
must also have the same trait, unless it has arisen due to an unlikely new mutation.
Examples of autosomal dominant traits and disorders are Huntington's disease and
achondroplasia.
• Autosomal recessive inheritance
• Autosomal recessive traits is one pattern of inheritance for a trait, disease, or disorder to
be passed on through families. For a recessive trait or disease to be displayed two copies
of the trait or disorder needs to be presented. The trait or gene will be located on a non-
sex chromosome. Because it takes two copies of a trait to display a trait, many people
can unknowingly be carriers of a disease. From an evolutionary perspective, a recessive
disease or trait can remain hidden for several generations before displaying the
phenotype. Examples of autosomal recessive disorders are albinism, cystic fibrosis.

17. • X-linked and Y-linked inheritance
• X-linked genes are found on the sex X chromosome. X-linked genes just like autosomal genes have both
dominant and recessive types. Recessive X-linked disorders are rarely seen in females and usually only affect
males. This is because males inherit their X chromosome and all X-linked genes will be inherited from the
maternal side. Fathers only pass on their Y chromosome to their sons, so no X-linked traits will be inherited
from father to son. Men cannot be carriers for recessive X linked traits, as they only have one X
chromosome, so any X linked trait inherited from the mother will show up
• Females express X-linked disorders when they are homozygous for the disorder and become carriers when
they are heterozygous. X-linked dominant inheritance will show the same phenotype as a heterozygote and
homozygote. Just like X-linked inheritance, there will be a lack of male-to-male inheritance, which makes it
distinguishable from autosomal traits. One example of an X-linked trait is Coffin–Lowry syndrome, which is
caused by a mutation in ribosomal protein gene. This mutation results in skeletal, craniofacial abnormalities,
mental retardation, and short stature.
• X chromosomes in females undergo a process known as X inactivation. X inactivation is when one of the two
X chromosomes in females is almost completely inactivated. It is important that this process occurs
otherwise a woman would produce twice the amount of normal X chromosome proteins. The mechanism
for X inactivation will occur during the embryonic stage. For people with disorders like trisomy X, where the
genotype has three X chromosomes, X-inactivation will inactivate all X chromosomes until there is only one X
chromosome active. Males with Klinefelter syndrome, who have an extra X chromosome, will also undergo X
inactivation to have only one completely active X chromosome.
• Y-linked inheritance occurs when a gene, trait, or disorder is transferred through the Y chromosome. Since Y
chromosomes can only be found in males, Y linked traits are only passed on from father to son. The testis
determining factor, which is located on the Y chromosome, determines the maleness of individuals. Besides
the maleness inherited in the Y-chromosome there are no other found Y-linked characteristics.

18. Pedigrees analysis
• A pedigree is a diagram showing the ancestral relationships and
transmission of genetic traits over several generations in a family.
Square symbols are almost always used to represent males, whilst
circles are used for females. Pedigrees are used to help detect many
different genetic diseases. A pedigree can also be used to help
determine the chances for a parent to produce an offspring with a
specific trait.

21. Down Syndrome - Trisomy 21