Heredity, Inheritance, and Variation

By: Victor Rea Oribe

Appreciate Similarities and Differences Among Organisms

What characteristics do you share among all human species?

What characteristics do you share among your siblings?

What characteristics do you share among your close relatives?

Which traits resemble your parent/s?

Which traits neither resemble your mom’s or your dad’s?

Have you been told that you have your mother’s nose or your grandmother’s eyebrows?

Examine closely the picture of a family, what characteristics do these siblings share in common?

People have been fascinated at how children will resemble their parents and vice versa.

As years went by, scientists began to search for more information on how these traits are passed on.

The passing of traits from parents to offspring is HEREDITY and the science that deals with the study of heredity is GENETICS.

The family picture shows how some traits of the parents blend into the traits of the offspring.

Can you name some of the traits?

Nature Versus NurtureHuman grow and develop according to the instructions in our GENES.

The Human Genome Project (HGP -1987-2003) was aimed to map out the sequence of the genes that will benefit many fields including medicine and human evolution.

The findings concluded that there are about 20,000 genes that shape how humans develop.

But despite this big number, it is very difficult to separate the relative influence of heredity and environment on an individual’s characteristics.

Human development is not just genetic.

The girl and boy in the picture may have had their father’s eye but got their mother’s lips.

The uniqueness is brought about by the combination of genes of their parents.

How will the environment influence their inherited traits?

The environment may influence an individual’s growth and development.

Height is determined by genes. Tall parents will likely produce tall children.

However, inadequate nutrition and lack of sleep may lead to stunted growth.

In similar way, intelligence which includes the speed of processing information has been studied to be genetically controlled.

But even if a person is predisposed to become intelligent, if the environment in which he or she grows up in is not stimulating, the person may not reach his full potential.

Similarly, a person may not have the perfect combination of genes for intelligence, but if the environment motivates and nurtures learning, the person may develop higher intelligence quotient.

Check Your Understanding1.Why does father or mother who paints well

will likely have children who are excellent painters as well?

2.Two identical twins (twins who look exactly alike and come from the same fertilized egg; same genetic make-up; same sex) were separated at birth. One was adopted by a rich family and the other by a poor sidewalk vendor. How will this scenario affect the twins?

Traits are observable characteristics determined by specific segments of DNA called GENES.

The DNA (deoxyribonucleic acid) is a double helix molecule, the thin strands of which are twisted around each other like a twisted ladder.

The sides of the ladder are made up of sugar and phosphate molecules and the steps of the ladder are made of nitrogen base pairs.

A GENE is a length of DNA that codes for a particular protein.

For example, a gene may code for the protein of your hair or a protein like insulin which controls the level of glucose or sugar in the blood.

The gene is responsible why you are different from everyone else alive today and even among those who lived in the past.

But it is also the same gene responsible why you shares some features of your family members.

Genes store the information needed to make the necessary proteins that will eventually lead to specific traits.

It determines how you will look outside and how your body will work inside.

The DNA are tightly coiled around special proteins called HISTONES and make up the chromosomes.

Chromosomes are found inside the nucleus of the cell.

Chromosomes replicate before a cell divides to make sure that each daughter cell will contain the complete set of chromosomes.

From Haploid to Diploid

All living things have the ability to reproduce in order to perpetuate their own kind.

But multi cellular organisms like human being go through a complex series of events in some cells of their body, specially in their sex cells or gametes, to maintain the chromosomes number of a particular species.

Chromosomes Number of Some Organisms

Organisms Diploid Number (2N)

Haploid (N)

Human 46 23Mosquito 6 3Housefly 12 6Cat 38 19Chimpanzee 48 24Dog 78 39Onion 16 8Rice 24 12Potato 48 24Cotton 52 26

The process by which sex cells divide and shuffle their genetic materials is called MEIOSIS.

Meiosis takes place in the sperm and egg cell of animal and in the anther and ovaries of plants.

It results in daughter cells which posses half the number of chromosomes of the parent cell.

This is necessary so that when sex cells unite during the fertilization, the original number of chromosomes is maintained.

Human being have 23 chromosomes in their GAMETES, and therefore contain only half of the complete set of chromosomes.

These sex cells are said to be HAPLOID. It is represented by letter n

During fertilization, the sperm carries only half the number of chromosomes (haploid) and unites with the egg cell which is also haploid.

Both father and mother contribute a copy of each gene to the resulting offspring.

Two sets of chromosomes from both parents now results in 46 chromosomes in the developing embryo.

The resulting zygote from the union of the egg and sperm now have a complete set of chromosomes and is said to be diploid, represented by 2n.

All cells of our body with the exception of the sperm cells contain the complete set of chromosomes.

The first 22 pairs are body chromosomes or autosomes and the 23 pair make up the sex chromosomes.

The sex chromosomes determine the sex of an individual.

Egg always carry an X sex chromosomes while the sperm may carry either X or a Y chromosomes.

The egg and the sperm carry one member of the sex chromosome pair.

If an X bearing sperm fertilizes an egg, the resulting offspring is a female (XX)

But if a Y-bearing sperm fertilizes and egg, the resulting offspring is a male (XY)

In short, the 44 chromosomes are autosomes and the 45th and 46th chromosomes are sex chromosomes.

In human, chromosomes number 1 is the biggest containing 8,000 genes and chromosomes 21 is the smallest with 300 genes.

Variation and Inherited Human Traits

The combination of the sex cells during fertilization results in VARIATION in the offspring.

Several factors influence this variation which actually happens during the meiotic process.

CROSSING OVER during the prophase stage of meiosis.

Crossing over allows the exchange of a part of a chromosomes with another part from its homologous or identical chromosomes.

It occurs at a number of different points on the same chromosomes, this leads to greater variation.

MUTATION or the permanent change of DNA sequence can also result in variation.

These changes may be brought about by environmental factors such as:

a) exposure to ultraviolet or nuclear radiationb) exposure to chemicals

It may also happen during the time when a cell copies its DNA before it divides.

If the mutation takes place in the gametes, they can readily passed on to the offspring.

Most genes have two or more variations called ALLELES.

For instance, the two alleles for hairline shape are straight and heart-shape hairline.

A child may inherit the same allele or two different alleles from his/her parents.

Two different alleles will interact in specific ways to cause a trait to be expressed.

Other inherited traits include:a) Free and attached airlobeb) Straight or the bent thumbc) Tongue rollingd) Color of eyese) Shape of lipsf) Color of hairg) Blood typeh) Diabetesi) Texture of the hair

The actual set of genes carried by an organism is its genotype and expression of manifestation of an organism, the gene type is called PHENOTYPE.

An individual is said to be HOMOZYGOUS if the alleles for a traits are identical or the same.

A person with two alleles for freckles is homozygous for that particular trait.

Similarly, if a person also has two alleles for no freckles, then he or she is homozygous as well.

An individual is said to be HETEROZYGOUS when the two inherited alleles are different for a particular gene.

In this case, an individual who possesses one allele for freckles and one allele for no freckles is heterozygous.

Alleles interact in a dominant or recessive manner.

A DOMINANT ALLELE expressed itself and masks the effect of the allele for the same traits.

For example, a person has an allele for free or hanging earlobes and one allele for attached earlobe, yet what was expressed in his physical appearance is free earlobes.

A RECESSIVE ALLELE on one hand is one expressed in the presence of the other allele.

Recessive allele can only be expressed when the organism is homozygous for the recessive alleles.

A trait is symbolized by one letter.for example: earlobe may be represented by

the letter E

Its alleles may be written in the following manner: EE, Ee, ee.

Its alleles may be written in the following manner: EE, Ee, ee.

Since free earlobes is dominant it is represented by a capital letter E, attached earlobe is recessive thus it is written as a small letter e.

a) homozygous dominant genotype EE = freeb) heterozygous Ee = freec) homozygous recessive ee = attached

Traits and Expression in Human

Traits Expression

1. Shape of face

Oval : dominant,

Square : recessive

2. Cleft in chin

No cleft: dominant

Cleft : recessive

Traits Expression

3. Hair curl (probably polygenic) Assume incomplete dominance

Curly: homozygous

Wavy: heterozygous

Straight: homozygous

4. Hairline

Widow’s peak: dominant

Straight hairline: recessive

Traits Expression

5. Eyebrow size

Broad: dominant

Slender: recessive

6. Eyebrow shape

Separated: dominant

Joined :recessive

Traits Expression

7. Eyelash length

Long: dominant

short: recessive

8. Dimples

Dimples: dominant

No dimples : recessive

Traits Expression

9. EarlobesFree lobe: dominant

Attached : recessive

10. Eye shape

Almond: dominant

Round :recessive

Traits Expression

11. Freckles Freckles: dominant

No freckles: recessive

12. Tongue rolling

Roller: dominant

Non roller : recessive

Traits Expression

13. Tongue folding

Inability: dominant

ability: recessive

14. Finger mid-digital hair

Hair : dominant

No hair :recessive

Traits Expression

15. Hitch hiker thumb

Straight thumb : dominant

Hitch-hiker thumb: recessive

16. Hair on Back hands

No Hair: dominant

Hair :recessive

Gene Mutation

Sometimes, an organism may appear with a genetic trait totally unlike anything that is seen in otjer members of the species.

This total unlikeness is a physical manifestation of changes at the biochemical level which are called MUTATIONS.

Mutations may either SOMATIC or GERM

SOMATIC MUTATION occurs in any body cells except the reproductive cells.

It is not passed on to the offspring and will cease to exists when the parent or organism dies.

GERM MUTATION occurs in reproductive cells and is transmitted to offspring.

It may be passed on from one generation to another.

Causes of Mutation

1. High energy radiationExposure to different kinds of rays : cosmic

rays, radiation from radioactive elements, X-ray, gamma rays, beta particles and ultraviolet rays.

High-energy radiation is one of the most frequent causes of mutations.

2. Chemical as mutagens (agent of mutation)among these mutagens are formaldehyde, nitrous acid, peroxide, mustard gas, marijuana plants

61 cannabinoids with the principal component delta-9-THC. This component has been traced as radioactive, and it takes 5 to 8 days for just half the THC in a single marijuana cigarette to clear from the body.

constant exposure to THC diminishes the capacity of individual cells to begin life according to genetic plan built into cellular molecules.

3. Induced mutationsOne form of induced mutation comes from recombinant DNA experiments.

Here, DNA from one kind of organism is treated with enzymes to isolate a specific sequence of genes.

These genes are then added to another kind of organism, and this added DNA recombines (with the help of some enzymes) with DNA already present in the recipients organisms.

Consequently, the organism can be considered a mutant

MUTATIONSChanges which can be

inherited

Changes in Chromosomes

Cause by Nature

Changes in Chromosomes

Changes in Chromosomes

Genetic Changes

NN Non-Mendelian InheritanceOur modern understanding of how traits may be inherited through generations comes from the principles proposed by Gregor Mendel in 1865.

His experiment on Pisum sativum, or pea plants led to the following principles:a) Independent assortment – traits are inherited

independent of each other.b) Dominance- when pure parents with opposite

traits are mated, the first generation shows only one traits (dominant). The other trait (recessive) is hidden

c) Segregation – when hybrid are crossed, the opposite traits are separated into different offspring in a ratio of 3:4 (dominant : recessive)

However, not all pattern of inheritance obey the principle of Mendelian genetics. In fact, many traits we observe are due to a combined expression of alleles

Non-Mendelian inheritance is a term that refer to any pattern of inheritance in which traits do not segregate in accordance with Mendel’s principle (that is, each parent contributes one of two possible alleles for a traits)

A. INCOMPLETE DOMINANCE and CODOMINANCE

Incomplete dominance occurs when one allele is unable to express its full phenotype in a homozygous individual.

This results to an individual with a phenotype that is intermediate to homozygous individual.

Red Carnation is dominant over white carnation.

Following Mendel’s principle of dominance, when these traits are crossed, the dominant red carnation should hide the recessive white.

Scientists after Mendel found out differently.

In 1760, Josef Kolreuter crossed pure red carnation (RR) and pure white carnation (WW) and produced pink carnation – an intermediate phenotype of red and white.

The phenotype of the offspring is a “blend” of the parent’s phenotype. This is an example of incomplete dominance.

CRCR CWCW

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CRCR CWCW

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F1 generation all CRCW

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CR CwFEMALEMALE

F2 generation 1:2:1

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Codominance is a situation in which both alleles are equally strong and both alleles are visible in the hybrid genotype.

An example of codominance is found in chickens.

When a white chicken is cross with black chicken, the result is not a grey chicken, but a chicken with both black and white feathers.

B. Multiple AllelesA gene with more than two alleles is said to have multiple alleles.

Many genes exist in multiple alleles.

A rabbit’s coat, for example has at least four different alleles

Hair color is determined by a single gene with a series of alleles, each resulting in different colors like alleles for black, brown, blond, etc.

C. Epistasis

The interaction between two or more genes to control a single phenotype result in an inheritance pattern called EPISTASIS.

It occurs when the action of one gene is modified by one or several other genes, which are sometimes called MODIFIER genes.

A gene can either mask or modify the phenotype controlled by the other gene.

Masking epistasis occurs when a gene at one locus masks the expression of a gene at the second locus so its phenotype is not expressed.

Modifying epistasis occurs when a gene at one locus modifies or change the expression of the phenotype of a gene at the second locus

The gene that does masking / modifying is referred to as epistatic, while the gene that is masked / modified is referred to as hypostatic.

Labrador’s coat color vary from yellow to black and is controlled by two dominant alleles, one for the presence of dark pigment (allele E) and another for the degree of pigmentation (allele B)

The coat of Labrador retrievers show masking epistasis.

Chaff Colour in Oat. In Oat, the chaff can have three colours - black, grey and white. Black colour (B-) is dominant over all others. In its absence grey colour (bbG-) is dominant over white (bbgg). The dominant gene of black colour (B) is epistatic over the alleles for grey and white chaff colour (G-and gg). When a pure black chaff producing plant (BBGG) is crossed with a pure white chaff producing plant (bbgg) the hybrids of F1 generation plants have black chaff (BbGg). On self breeding the resultant plants of F2 generation have three types of chaff in the ratio of 12 black : 3 grey : 1 white.

Fruit Color in Cucurbita pepo. In Summer Squash or Cucurbita pepo, there are three types of fruit color- yellow, green and white. White color is dominant over other colors while yellow is dominant over green. Yellow color is formed only when the dominant epistatic gene is represented by its recessive allele (w). When the hypostatic gene is also recessive (y), the color of the fruit is green.

D. Sex-linked TraitsAfter Mendel’s garden peas, an American geneticist Thomas Hunt Morgan studied genetic variations in drosophila melanogaster (fruit fly).

In 1910, while examining a large number of drosophila, Morgan found one that had white eyes instead of the normal red ones; the fly was a male.

Morgan had the white-eyed male mate with a re-eyed female.

All of the offspring in the first generation had red eye.

He then mated flies from this generation. As a result, three-fourths of the second generation offspring had red eyes and one-fourth had white eyes.

The results seemingly conformed with the results Mendel observed in garden peas.

However, there was one striking difference, that is, all of the white-eyed flies were males.

Legend W : Dominant gene for red eyes

XW Xw Female, pure red eyed

XW Xo Male, red eyed (W cover O)

White-eyed male X Red-eyed female (pure)XW Xo XW Xw

FEMALE

MALE

XW Xw

XW

Xo

XW XW XW Xw

Xw X0 Xw X0

Female, red Female, red

Male, red Male, red

Resulting F1 Genotype: all female are carriers of white-eyes recessive gene

Phenotype: All males are normal; red- eyed

All females appear red- eyed because W is dominant.

Legend w : Recessive gene for white eyes

XW Xw Female, hybrid, red eyed (W dominant

Xw Xo Male, white eyed (O cannot cover w)

Red-eyed male X Red-eyed female (hybrid)Xw Xo XW Xw

FEMALE

MALE

XW Xw

Xw

Xo

XW Xw Xw Xw

XW X0 Xw X0

Female, red Female, red

Male, red Male, red

Resulting F1 Genotype: One-half of the females are normal, pure red-eyed

One-half of the females appear red but are carriers

Phenotype: One-half of the males are normal; red eyed