Genetics Powerpoint – Part 1


*Humans have 23 pairs of chromosomes with 1 of each pair coming from Mom, the other from Dad.  (picture of 23 pairs similar to tactile karotype picture)


u   Chromosomes consist of long strands of DNA, whose structure is often described as a double helix or twisted ladder.

u   Genes are portions of this “twisted ladder”. A particular chromosome may contain over 1000 genes down its length.


*“Nucleotide Base Pairs” Form the “Rungs” of the Double Strand DNA “Ladder”

(picture of pairs of nucleotide bases making up the rungs of the DNA ladder like that in atlas)


u   Human genome contains about 3 billion of these nucleotide base pair “rungs”. Sequences of these base pairs makeup our   ~ 30,000 genes.


Adenine - Thymine

Guanine – Cytosine

These 4 are the “letters” of our genetic alphabet


u *Link to map of the genes on chromosome 19

(picture of  banded chromosome 19 similar to that in tactile atlas)


u  During “expression” of genes portions of the DNA ladder split lengthwise down the middle. “Rungs” of one strand (half ladder) serve as a template to make RNA. RNA then serves as a template for building proteins. Every 3 rung combination (like adenine, adenine, guanine) is the instruction for adding an amino acid to the new protein.

Mendelian Genetics

u Studied simple traits and how they are passed on to offspring

u 2 possible “alleles” or variants of the gene for simple traits

u The combination one has (1 from mom, 1 from dad) = your “genotype”

u If you have 2 matching alleles = you are “homozygous”

u If you have 2 different alleles = you are “heterozygous”


*Heterozygous  Example

(picture of a pair of chromosomes, each with the locus of the “flower color gene” labeled. One chromosome contains a gene for purple flowers, the other a gene for white flowers, so this individual plant is “heterozygous” for flower color.


Mendelian Genetics

u  If you are heterozygous for a trait, the “stronger “ or “dominant” allele is “expressed” (seen in individual’s appearance)

u  The traits that are expressed or seen are your “phenotype”- e.g. brown eyed

u  The  weaker allele that is not expressed is the “recessive” allele. Even though not expressed, it is part of your “genotype” & can be passed on to your kids.

u  Dominant alleles indicated by upper-case letters

u  More than 2 varieties of alleles exist for some traits


Punnett’s Square

u  A tool for visualizing the possible genotypes of offspring given the genotypes of the 2 parents (or, conversely, to help you figure out the genotype of a parent from the genotypes of the offspring)

u  Letters used are arbitrary- just a tool to think about dominant (upper case) and recessive (lower case) alleles

*Punnett’s square is a square or rectangle divided into 4 quadrants with one parents genotype written above the left and right quadrants – for instance Br above the left side and bl above the right side if Mom had brown eyes but we knew she carried a blue gene too. The other parent’s genotype is recorded outside the square on the left, next to the upper and lower quadrants.  E.g. maybe Dad has 2 blue genes. By looking at the genetic contributions possible from each parent, each  quadrant within the square can be filled in

with the possible genotypes of any offspring of the couple. For example the upper left quadrant (just under the label indicating Mom has a Br gene) would be filled in with one genotype possible – that the child will get a Br from Mom and 1 of the bl from Dad.

The quadrant below, in this example would look the same – a Br from Mom and a bl from Dad (but representing the other of his bl genes, even though the effect or outcome will be the same.). The top right quadrant (just under the label indicating Mom carries a blue gene) would show the child possibly getting that bl gene as well as a bl gene from Dad (thus having blue eyes). The lower right quadrant would look the same (bl – bl) but would again represent the child getting Mom’s bl gene and the other of Dad’s blue gene.

Looking at the total square we can see these parents have a 50-50 chance of having a Br-bl child and 50-50 chance of having a bl-bl child each time they get pregnant.


Another Example

u  *This is a Punnet square for the situation where both parents are heterozygous for a trait.If both parents are heterozygous for a gene, they may have kids with all of these possible genotype: ¼ would be expected to have 2 dominant genes, ¼ would be expected to have 2 recessive genes, and 2/4 would be expected to be heterozygous (each of those fourths refers to one of the quadrants)


Genetics Assignment

u Explore family member phenotypes and predict the genotypes for 6 traits, such as those on this web page:


Genetics 2



Nucleotide “Rungs” of DNA

Human genome contains about 3 billion of these nucleotide base pair “rungs”  which make up about 30,000 genes.

We share 99.8% genes in common with other humans, 98% genes in common with chimps, and lots of genes in common with all living things (e.g. ½ the banana genome found in the human genome)


Shuffling the Deck

After the initial joining of 23 chromosomes from Mom and 23 from Dad (when the egg is fertilized), the resulting 23 pairs of chromosomes in the new individual are going to replicate each time the developing cells divide. In these later cellss the chromosomes of offspring don’t always have one that’s completely from dad and one completely from mom because

*during replication there may be  “crossing over” of the “arms” of a pair of chromosomes and exchange of DNA (e.g. the end of the arm from the point where Dad’s chromosome crosses Mom’s changes places with the end of the arm on Mom’s chromosome. Now each member of the pair contains some genetic material from each parent.

   First 22  pairs are autosomes (matched pairs); last is the pair of sex chromosomes


*X and Y Chromosomes


Sex-Linked Genes (X-linked genes)

In the case of genes located on the X chromosome, females (who have 2 X’s) would have the usual 2 alleles for each gene, one on each X.

 Males, however, only have 1 X chromosome and a much smaller Y containing less than 1/4 as many genes(mostly related to testes & body size). So many genes on the X do not have a corresponding allele on the Y. If a male inherits a recessive gene at one of these positions on his X, it will be expressed because he will ONLY have the recessive allele. *Example – X-linked recessive red/green color blindness gene, X-linked hemophilia                 


Remember – genes provide the templates for building proteins

If females have functioning genes on both X’s and males only have the genes on 1 X, you might expect half as much of the X related proteins to be produced in males. But in females there is random inactivation of 1 X in each cell!



Types of Genes

Structural genes - contain the instructions for building proteins (via RNA & ribosomes)

Operator genes - determine whether particular structural genes will be active or “expressed” or not

Operator genes can be influenced by signals from the bodily or external environment.

This is one example of regulation of gene expression.



Regulation of Gene Expression

All cells of an individual contain the same chromosomes, yet the genes that “play off” or are expressed vary in each cell and at different times in the individual’s life. This is why, for example, despite the fact that they contain the same chromosomes and genes, skin cells are different from brain cells which are different from muscle cells, etc. A different combination of genes triggers differences in the structure and function of each due to various types of gene regulation.  Some examples: genes may be blocked from being expressed until a certain compound or environmental condition is present, genes may be switched on or off at different times. Ease of RNA binding may be increased or decreased.


Example: Sex-Limited Genes

Some genes, present in both sexes, are only expressed if exposed to particular sex hormone.

Example: Any of us may carry the gene for male pattern baldness, but it is only expressed in the presence of significant levels of androgens. Usually only those with testes will have these levels (later in life) and then the gene will be switched on and expressed. But female bodybuilders taking anabolic steroids can inadvertantly switch on this gene and then show male pattern baldness if they have that gene.


So, In Genetics,
What You “Get” Is Not Necessarily What You “See”

don’t usually  “see” the effects of a recessive gene unless you have a pair of them;

 a sex limited gene will only be expressed under certain hormonal conditions;

expression of many genes is “regulated” (turned on or off) by other factors







An estimate of the amount of the variance in the behavior/characteristics of a specific group/population is due to genetic variation.

Heritability ranges from 0 (none of variance due to heredity) to 1 (all of the variance due to heredity)

Heritability depends on the group or population studied



Methods for Studying Heritability in Humans

Comparing the similiarity of pairs of identical twins vs. the similarity of pairs fraternal twins

Comparing the similarity of child/biological parent vs child/adopted parents

BUT: these studies sometimes overestimate heritability


Behavior Genetics

Field of research focused on understanding the relative contributions of heredity and environment (and their interaction) on behavior and psychological functioning

Other research methods used: inbred strains, selective breeding/artificial selection , genome mapping and manipulation

Overestimating Heritability

Identical twins not only share their genetic makeup in common: they often share the same blood supply & chorion in utero, whereas fraternal twins typically have separate chorions.

Identical twins may be raised & reacted to more identically than fraternal twins.

Even in studies looking at twins raised apart, adoptive parents are screened to have similar characteristics, so “environments” not that different

So some portion of twin similarity may be due to environment


A change, over generations, in the frequencies of various genes in a population

Observations Leading to Darwin’s Theory of Evolution

Evidence of the gradual change in species over time

Striking similarities between species suggesting common ancestry, yet differences between related species adapting to different environments

Major changes in species via selective breeding

Selective Breeding

Selective breeding  ( or artificial selection) can lead to dramatic behavioral or physical differences in different strains of the same species.

But in some cases the interaction of genetics and environmental factors can cause the differences to disappear- e.g. Tryon maze-bright and maze-dull rats.



Phenylketonuria (PKU)

Metabolic disorder transmitted by recessive gene on Chromosome 12, causing a lack of the enzyme phenylalanine hydroxylase and a toxic buildup of phenylalanine.

This impairs brain development & causes retardation, hyperactivity, hyperirritability, possible seizures. Also low levels of dopamine.

Characteristics of PKU

About 1 in a 100 Caucasians (especially Scots and Irish), fewer Asians, and very few Blacks carry the recessive gene.

1 in 10,000 babies born with PKU (those who get the recessive gene from each of their parents).

But remember:

Even in genetic disorders “Nature” may interact with “nurture”

PKU’s effects depend on the environment (diet consumed)

With a special diet most of PKU’s effects are prevented

Most states require screening of newborns


Environmental Influence

Controlling diet (low phenylalanine) greatly alters effects of PKU.

A more normal diet may be eaten more safely after the “sensitive period”*

*But: PKU kids show “frontal lobe” type cognitive deficits (like impulse control) despite diet - may be due to  higher than normal phenylalanine levels even on the diet or low DA.


Huntington’s Disease

DNA mutation produces excess “CAG repeats” in the gene’s “instructions”

36-250 instead of  usual 29 or fewer, resulting in an abnormal form of protein known as huntingtin.

The more repeats, the earlier symptoms appear.

# of repeats can increase across generations, especially in kids inheriting gene from father

Brain damage may be due to decrease in normal protein + adverse effects of abnormal protein on critical growth factors keeping cells alive


*CAG Repeats & Age of Symptom Onset