GENES AND EXPERIENCE:Mendelian Genetics, DNA, Sex Influenced Traits

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Neurological Basis of Behavior (PSY - 610)

Lesson 09

GENES AND EXPERIENCE

Objective:

· To understand the Brain behavior relationships from the perspectives of biological systems.

In this section of the module the students would go through and understand Biological systems- their

genetic basis and similarities with other animals. Mendelian Genetics. Where is the genetic

programming of behavior (if any) Watson Crick Model. What are the similarities and differences

between species, progenies of higher order animals, including homosapien?

Mendelian Genetics:

Based on a series of studies which began accidentally when an Austrian Monk Gregor Johannes Mendel

planted sweet peas outside the monastery walls. He noticed a certain pattern in the colors and then he

manipulated the plantation, wherein the Mendelian genetic began through Pea studies. However it

appears that I may not have been that accidental as Mendel was following the readings of Lamarkian

and Darwinian theories.

In 1866, he drew the following conclusions:

Heredity is transmitted by hereditable units.

When both parents transmit the same trait it is expressed in the young offspring, but when the

two parents have a different gene, a hybrid form is established, which carries a unique

combination of traits.

Dominant traits are traits that are expressed, whereas the recessive traits are inherited but not

expressed (they remain in the background and get expressed when the offspring gets a recessive

gene from both parents.

Experience cannot affect the hereditary unit (genes).

For fifty years, Mendel's work went unnoticed, but came into eminence in the 1900's.

Hugo de Vries added the concept of mutations which are characteristics accidental created in the genetic

material and then transmitted in the mutated form to the offspring.

Mutation is defined as an inheritable change in the genetic material, not reshuffling of the old gene

material/combination. It could be genetic mutation: change in only one gene or chromosomal mutation:

which is change in the chromosomal combinations.

Immediately after fertilization the cells start dividing. When the cells divide, the chromosomes also

reproduce in the zygote (fertilized cell). Cell division is meiosis (within each cell of parents which

produces the gamates), mitosis is the process of cell division which takes place in the zygote).

The two genes at one location are known as alleles. If the alleles are same at the same location (from

two parents ), then the newly formed zygote is homozygous for that gene. However, when two different

paris of genes or alleles in the same location i.e. a different allele from the father or mother, then the

gene is heterozygous ( where one become dominant and the other recessive)

DNA

Heredity is transferred through DNA: deoxyribonucleic acid. DNA is found only in the cell nucleus

(and it stays there), it is most stable, and is self-replicating. It is composed of basic components i.e.

Neurological Basis of Behavior (PSY - 610)

simple sugars (deoxyribose) phosphates and four nucleotide bases: Adenine, Guanine, Thymine and

Cytosine),

Watson and Crick broke the DNA molecule code, which is double helix model. Each consists of two

strands wound around each other like a rope of two strands and these are attached to chains of

phosphates and deoxyribose (like a ladder). The four nucleotide bases, adenine, thymine, guanine and

cytosine are in permanent combinations where adenine always bonds with thymine and guanine with

cytosine (AT, GC).

DNA replicates itself during cell division, the sequence is remembered, and copies made. When the cell

is replicating the DNA strand unwinds itself. One strand remains stable while the other breaks up. One

pair of the bases in these strands also separate, while one base of each pair remains attached to each

strand (look at the figure to see how it opens up). A continuous sugar phosphate sugar phosphate

chain supports this double helix strand. Two identical strands are then created. This self replicating

process continues (sometimes things can go wrong- mutations can occur to change the DNA sequence).

How is heredity transferred when DNA does not leave the nucleus, how is heredity transferred?

The required proteins are manufactured and metabolized in the ribosomes, the message of genetic code

and materials needed is sent through the Ribonucleic acid which is similar to DNA. Therefore the code

can be transferred and transcripted by the RNA. The RNA which does this is called the messenger RNA

or mRNA which carries the genetic code from the nucleus of the cell to the ribosomes in the cell soma.

The Transfer RNA t RNA transfers the needed amino acids for manufacturing to the ribosomes. The

kinds of proteins made and when they are made determine whether the organism is male or female,

human or ape or drosophilia, tree or flower. If flower what color? What height etc?

Behaviors and Genes

Behaviour genetics is the specialization which aims to identify the genetic basis of behavior.

Genes: are arranged in a linear sequence along the chromosome.

Alleles: are genes occurring a given location Homozygous if both pairs of genes at same location carry

same (brown eyes, heterozygous if different genes on same site. If different then one is dominant, and

the other remains recessive.

Dominance is the tendency for one type of characteristic to express itself; however, this is affected by a

number of factors such as sex e.g. the same gene may be dominant for males and recessive for females

(baldness).

The continuous shuffling and reshuffling of chromosomes during reproduction leads to genetic variation

in the population on which the natural selection can work.

Example:

If we have a dominant homozygous gene of sickle cell anemia in African American the receiver cannot

survive beyond infancy ( in this case both parent had to have recessive genes for this condition where

the

red blood cell Is shaped like a sickle and does not have a normal shape

Sickle cell was a beneficial selection in Africa, especially for malaria infested areas, where this lead to

adaptation and ensure greater survival rates. This did not work when they moved to colder climates such

Neurological Basis of Behavior (PSY - 610)

as the USA where greater levels oxygen was needed and the sickle shaped red blood cell was deficient.

The survival in high altitudes is challenging even if the trait is recessive.

Genetic variability vs. In breeding

Genetic variability is important as it increases gene pool, thereby making a wider range of different

genes available. This increases the chances of transmission of healthier genes and lowers the chances of

bad recessive genes from showing up as in cousin marriages where because of in breeding behavioral

and other defects are expressed and have devastating effects on families. Why? The gene pool is

restricted and the same gene pool is used again and again. Thus genetic variability is beneficial as it can

a) lead to the development of a new species through sexual recombinations, natural selection and even

mutations, b) ensure survival of existing species when the environment changes, there is need to change/

to adapt to the changes in the environment e.g. African slaves with sickle cell anemia in cold America,

those survived who were heterozygous for this gene.

The inheritance of traits is also linked to the sex chromosome or the distribution of sex chromosomes (

gene follow the distribution of the sex chromosomes). There are several different ways in which the

transmission follows the sex chromosomal distribution.

1 Sex Linked traits

This means that the sex inherited is also linked to traits that are inherited. When traits are located on a

gene which is carried on the X chromosome it is said to be sex linked. The chromosome Y is small and

carries very few genes. In sex linked trait, the X chromosome carries genes for the trait along with the

sex. Thus it is determined by following the X distribution. This does not mean that these characteristics

are linked to male or female sex, it is only X linked, thus it would go wherever the X on which this gene

is located, goes). Sex linked is X linked.

As we already know that the Male has one of each XY chromosme, and females have two XX

chromosomes, if there are two XX's then trait carried by the X can be overcome in competition (cannot

express itself). However, in males since there is only one X, so the defective gene expresses itself. In

males the X chromosome contains genes which are lacking in Y; therefore the traits on it are expressed.

Thus in Males these characteristics are expressed, whereas in females there are not. This can happen

only if both XX's have it, then the trait is, expressed. The following traits are sex linked:

Hemophilia,

Colorblindness,

Huntington's chorea

Turner's syndrome (this has XO, i.e. one missing sex chromosome, no YO found yet).

These are transmitted from the mother's X to sons it is expressed and if transmitted to daughter's

recessive it remains recessive.

Two important characteristics of Sex linked traits:

a) The incidence is higher in males

b) it is never transmitted from male parent to male offspring

2. Sex Influenced Traits

The trait is inherited and transmitted equally by both sexes; however sex determines the dominance,

(gene dominant in one sex and remains recessive in the other). The two famous examples are the white

Neurological Basis of Behavior (PSY - 610)

forelock, and baldness.

Characteristics of sex influenced traits:

a) It is more common in men than women (not sex linked as the father's gene is transmitted equally to

sons and daughters, whereas in sex linked father did not pass on any genes to sons).

b) The trait shows up in men when neither mother nor father shows it (double recessive).

c) Shows up in all the sons of a woman who has the trait.

3. Sex Limited

When traits are expressed in one but not in the other sex. These characteristics are carried by the sex

genes as well as other genes. However these require the right amount of hormones for expression. It is

not necessarily complete dominance always; it can be incomplete sex limited trait. Reproduction only

is only found in females, and in men there is growth of hair on ears with age and beards. The sex not

genes which condition the expression of these traits, Montague (1954) reported that albinism was more

in males 95 as compared to 5 females (incomplete sex limited). Similarly alkeptonuria (black urine) a

disorder of the breakdown of phenylalanine also occurs more in males than females

References:

1. Kalat J.W (1998) Biological Psychology Brooks/ Cole Publishing

2. Carlson N.R. (2005) Foundations of Physiological Psychology Allyn and Bacon, Boston

3. Pinel, John P.J. (2003) Biopsychology (5th edition) Allyn and Bacon Singapore

4 Bloom F, Nelson and Lazerson (2001), Behavioral Neuroscience: Brain, Mind and Behaviors (3rd

edition) Worth Publishers New York

5. Bridgeman, B (1988) The Biology of Behaviour and Mind. John Wiley and Sons New York

Note: References #2, 3, and 4 are followed most closely, as they have been used in teaching as well;

further individual references/pages are also given on the power points of each lesson

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