linkage · 2020. 4. 15. · • linkage without crossing over creates only parental (non crossover)...

LINKAGE

•In eukaryotic species, each linear chromosome containsa long piece of DNA

•A typical chromosome contains many hundred or even afew thousand different genes

•The term linkage has two related meanings

1. Two or more genes can be located on the samechromosome

2. Genes that are close together tend to be transmitted asa unit

Happy with my class, give a missed call at 080-3951-0319, If you are unhappy give missed call at 080-3951-0320

• Linkage happens when genes don’t assortindependently and genes linked on the samechromosome segregate together.

• Linkage without crossing over creates only parental(non crossover) gametes.

• Chromosomes are called linkage groups

– They contain a group of genes that are linked together


• The number of linkage groups is the number of types of chromosomes of the species

– For example, in humans

• 23 linkage group in humans

• 7 linkage groups in sweet peas

• 4 linkage groups in drosophila melanogaster


No Linkage: Independent Assortment


Linkage


Features of Linkage

• Genes that show linkage are situated on the samechromosome

• Genes are arranged in a linear fashion in thechromosome i.e linkage of genes is linear

• Distance between linked genes is inverselyproportional to the strength of linkage

• The genes that are closely located show stronglinkage whereas those which are widely separatedhave more chance to get separated by crossingover (weak linkage)


• Linked genes remain in their original combination during course of inheritance

• Linked genes show two types of arrangement

– Cis arrangement of genes

– Trans arrangement of genes


Bateson and Punnett Discovered Two Traits That Did Not Assort

Independently• In 1905, William Bateson and Reginald Punnett

conducted a cross in sweet pea involving two differenttraits

– Flower color and pollen shape (Purple flower andlong grain pollen)

• This is a dihybrid cross that is expected to yield a 9:3:3:1phenotypic ratio in the F2 generation

– However, Bateson and Punnett obtained surprisingresults


Parents P

Genotype PpLlGenes not linked

Genotype PpLlGenes linked

Self-cross Self-cross

p

L l

P

L

p

l

Figure 5.10


Parents P



Self-cross

F1

Self-cross

p

L l

P

L

p

l


Parents P



Self-cross

F1

Self-cross

p

L l

P

L

p

l

Female gametesPL Pl pL pl

Malegametes

PL

Pl

pL

pl

Female gametesPL pl

Malegametes

PL

pl


Parents P



Self-cross

F1

Self-cross

p

L l

P

L

p

l


PPLL PPLl PpLL PpLl

PPLl PPll PpLl Ppll

PpLL PpLl ppLL ppLl

PpLl Ppll ppLl ppll

Malegametes

PL

Pl

pL

pl

Female gametesPL pl

PPLL PpLl

PpLl ppll

Malegametes

PL

pl


Parents P



Self-cross

F1

Phenotypic ratio 3:

Self-cross

p

L l

P

L

p

l


PPLL PPLl PpLL PpLl

PPLl PPll PpLl Ppll

PpLL PpLl ppLL ppLl

PpLl Ppll ppLl ppll

Malegametes

PL

Pl

pL

pl

Female gametesPL pl

PPLL PpLl

PpLl ppll

Malegametes

PL

pl

Phenotypic ratio 9:3Happy with my class, give a missed call at 080-3951-0319, If you are unhappy give missed call at 080-3951-0320

Parents P



Self-cross

F1

Phenotypic ratio 3:

Self-cross

p

L l

P

L

p

l


PPLL PPLl PpLL PpLl

PPLl PPll PpLl Ppll

PpLL PpLl ppLL ppLl

PpLl Ppll ppLl ppll

Malegametes

PL

Pl

pL

pl

Female gametesPL pl

PPLL PpLl

PpLl ppll

Malegametes

PL

pl

Phenotypic ratio 9:3:3Happy with my class, give a missed call at 080-3951-0319, If you are unhappy give missed call at 080-3951-0320

Parents P



Self-cross

F1

Phenotypic ratio 3:1

Self-cross

p

L l

P

L

p

l


PPLL PPLl PpLL PpLl

PPLl PPll PpLl Ppll

PpLL PpLl ppLL ppLl

PpLl Ppll ppLl ppll

Malegametes

PL

Pl

pL

pl

Female gametesPL pl

PPLL PpLl

PpLl ppll

Malegametes

PL

pl

Phenotypic ratio 9:3:3:1Happy with my class, give a missed call at 080-3951-0319, If you are unhappy give missed call at 080-3951-0320

CROSSING OVER• Crossing Over involves reciprocal exchange of

chromosome segments between homologs; increasesgenetic variation (recombination).

• Linkage with crossing over creates parental gametes and recombinant (crossover) gametes.

• Interlocus distance is proportional to the degree of crossing over between.

– Little or no crossing over in close genes.

– Frequent, even multiple crossovers between distant genes.


Linkage with Recombination


Crossing Over May Disrupt Linkage


Crossing Over May Produce Recombinant Phenotypes

• In diploid eukaryotic species, linkage can be altered during meiosis as a result of crossing over

• Crossing over

– Occurs during prophase I of meiosis at the bivalentstage

– Non-sister chromatids of homologous chromosomes exchange DNA segments


What is Crossing Over?

The exchange of chromosomal segments between two non- sister

chromatids


Crossing Over


When Does it Happen?

During Prophase I of Meiosis

Meiosis increases genetic diversity in a speciesMitosis creates genetically identical daughter cells


Where does Crossing Over Occur?

Genetic swapping occurs between paired homologous chromosomes in our sex

cells—The Egg and Sperm


Egg and Sperm

• Chromosomes pair• Chromosomes may swap genetic material• New genetic material has been acquired from the

other homologue

• NOTE: Genes that have a tendency to remain together during crossing over are said to be linked


Why Does Crossing Over Occur?

To provide genetic variation during meiosis

Genes that are far apart have a GREATER chance of crossing over

Genes that are closer have a LESS LIKELY chance of crossing over


Diploid cell afterchromosome replication

Meiosis

Possible haploid cells(a) Without crossing over, linked alleles

segregate together.

ba

ba

ba

BA

ba

BA

Diploid cell afterchromosome replication

Meiosis

Possible haploid cells(b) Crossing over can reassort linked

alleles.

bA

BA

ba

Ba

BA

BA

BA

ba


What is Sex Linkage?

• In addition to their role in determining sex, the sexchromosomes have genes for many characters.

• Genes located on a sex chromosome are called sexlinked genes.

• In humans the term usually refers to X-linkedcharacters: genes located only on X chromosomes.

• Fathers can pass X-linked alleles to their daughters,but not sons.

• Mothers can pass sex-linked alleles to both sons anddaughters.


Thomas Hunt Morgan – (the father of modern genetics)

He found genes located on the X chromosome that regulate fruit fly eye color.

Genes on the Xchromosome aresaid to be X-linked


2) When he crossed two F1 red-eyed flies,

he got three red-eyed flies to every one

white-eyed fly. ALSO-the white-eyed flies

were male.


As stated by Morgan the gene for eye color is located on the X chromosome.

R = red color r = …white

complelely color dominant over the…


• Because females get a second chance at a dominant gene, they’re usually red-eyed.

• Males only get one chance at a dominant.

• Males are more likely to express recessive traits if the gene is on the X chromosome. (hemophilia, color-blindness, muscular dystrophy.)

Male fruit flieshave white eyesmore often than

females


Red-green colour blindness

• X chromosome has a locus for colour vision with two alleles:

– XN = Normal colour vision

– Xn = Red-green colour blindness

• Y chromosome does not have a colour vision locus.

• If a male receives the Xn allele he will have impaired colour vision, whereas a female with XNXn will not.


Red-green colour blindness

ParentalPhenotypes Carrier Female x Normal MaleGenotypes XNXn XNYGametes

Offspring 1Genotypes

Phenotypes Normal Female : Carrier Female : Normal Male : Colour blind Male

1 : 1 : 1 : 1

XN Xn XN Y

Female Gametes

XN Xn

Male Gametes

XN XN XN XN Xn

Y XN Y Xn Y


Red-green colour blindnessParental

Phenotypes Normal Female x Color Blind MaleGenotypes XNXN XnYGametes


Phenotypes

Carrier Female : Normal Male 2 : 2

XN XN Xn Y

Female Gametes

XN XN

Male Gametes

Xn XN Xn XN Xn

Y XN Y Xn Y

All children have normal vision, daughters are carriersPhenotypes Clour blind woman x Normal Male


ParentalPhenotypes Clour blind woman x Normal MaleGenotypes XnXn XNYGametes


Xn Xn XN Y

Female Gametes

Xn Xn

Male Gametes

XN XN Xn XN Xn

Y Xn Y Xn YPhenotypes

Carrier Female : Color Blind Male 2 : 2

All sons are color blind, daughters are carriersHappy with my class, give a missed call at 080-3951-0319, If you are unhappy give missed call at 080-3951-0320

Sample Problem:

• A man with RGCBmarries a woman that isa carrier for RGCB.Determine thepossibility of their sonbeing born with RGCB.

• What is required?

– Possibility of son possessing RGCB

• What is given:

– Mother XBXb

– Father XbY


Solution:

Xb Y

XB Xb XB Y

Xb Xb Xb Y

XB

Xb

The son will have a 50% chance of being born with RGCBHappy with my class, give a missed call at 080-3951-0319, If you are unhappy give missed call at 080-3951-0320

Sex-influenced characteristics

• Sex-influenced characteristics are determined byautosomal genes and are inherited according toMendel’s principles, but they are expresseddifferently in males and females.

• Its expression is influenced by gender (presence ofhormones such as: estrogen, progesterone,testosterone, etc.).

• Sex-influenced traits vary in the degree of thephenotypic expression.


• An allele may be expressed as a dominant in one sexand a recessive in the other

• The trait has higher penetrance in one of the sexes.

• Difference in the ways in which the two genderexpress their genes

Example: the presence of a beard on some goats isdetermined by an autosomal gene Bb that is dominantin males and recessive in females.


Example: Pattern Baldness

• It can occur in both males and females, but isinfluenced by the hormone testosterone.


The combination of alleles lead to different phenotypic expressions depending on gender.


What is the probability that you will be bald if your father is homozygous and balding, and your mother is homozygous and not balding?

Father = B’B’ x Mother = BB

• All offspring are BB’

• If you are male, then you will be bald.

• If you are female, then you will not be bald.


Peacock plumageThe plumage of a male peacock is highlydecorated and colored, while the plumage of afemale peahen is dull by comparison.


• Rheumatoid arthritis occurs more often in femalesthan males due to the presence of estrogen. Aheterozygous woman marries a heterozygous male.RR would cause the condition in both sexes. Ahomozygous recessive, rr, genotype would preventthe disorder in both sexes


Sex-limited characteristics

• Sex-limited characteristics is encoded by autosomalgenes that are expressed in only one sex.

• The trait has zero penetrance in the other sex.

• Sex-limited traits- characteristic only appears (ordevelops) in one of the sexes.


• Example: Ovary development in females; Spermdevelopment in males

• Example: genes that control milk yield and quality indairy cattle are present both in bulls and cows buttheir effects are only expressed in the female cattle


Interaction between Sex and Heredity

Sex-limited characteristic

Determined by autosomal genes

Expressed only in one gender

Cock feathering, autosomal recessive Expressed only in males

Cock-feathered male Hen-feathered female Hen-feathered male


Sex-limited Traits

• The pattern of hen feathering depends on theproduction of sex hormones

• If the single ovary is surgically removed froma newly hatched “hh” female

• She will develop “cock” feathering and lookindistinguishable from a male


Sex – limited traits Sex influenced traits

Traits expressed only in one sex Traits appear in both sexes, but occur more in one sex.

Genes may be present in both sexes, but expressed in only one.

Ex: pattern baldness in females it appears as thinning of hair, but in males it induces

loss of hair. Ex: breast production, ovarian development female specific.

Facial hair, sperm productionMale specific

The pattern baldness to be completely active need male hormone Testosterone.


Pseudo-dominance

• When King is absent form palace…who acts asthe king?

• Recessive genes start behaving as dominant !!

• Hemizygous.


4 4

• Only males are affected

• Affected males pass the disease gene to all their sons and to none of their daughters

Y-linked (Holandric)


Different types of Sex Influences on heredity

Genetic phenomenon Phenotype determined by

Sex-linked characteristic Genes located on sex chromosome

Sex-influenced characteristics Genes on autosomal chromosomes that are more rapidly expressed in one sex

Sex-limited characteristic Autosomal genes whose expression is limited to one sex

Genomic imprinting Genes whose expression is affected by the sex of transmitting parent


Genetic anticipation

• Exhibit a progressively earlier age of onsetand an increased severity of the disorder ineach successive generation.

• Huntington disease

• Molecular basis:

Anticipation is due to the expansion of atrinucleotide repeat sequence in the DNA.


Fact File

• The seven pairs of characters studied by Mendel areactually present on four pairs of chromosomes i.e on1, 4, 5 and 7th chromosome

• Mendel’s work is republished in 1901 by deVries inthe journal “FLORA”

• In Mendelian crosses the number of phenotypiccombinations is derived using the formula 2n and thenumber of genotypic combinations by using formula3n, where n represents the number of traits studied


• Eg. In dihybrid cross

• n = 2, phenotypic combinations 2n = 22=4 (9:3:3:1)

• Genotypic combinations 3n = 32=9 (1:2:2:4:1:2:1:2:1)

• Types of gametes in a given genotype is 2n , where nrepresents the heterozygous genotype


linkage · 2020. 4. 15. · • linkage without crossing over creates only parental (non crossover)...

Documents