Microsoft PowerPoint - Week_5_Mendel_PPT_ Lecture

Mendelian Genetics:

Principles of Inheritance and Mendelian Traits

First, a bit of Review…

• Remember the structure of DNA?• And how it relates to a GENE?• Check out the next slides to get a

different perspective from your textbook on these concepts…

DNA : deoxyribonucleic acid

• DNA is found in the nucleus of a cell and carries the genetic instructions for making living organisms.

• The structural arrangement of DNA, which looks something like an immensely long ladder twisted into a helix, or coil.

• The sides of the "ladder" are formed by a backbone of sugar and phosphate molecules, and the "rungs" consist of nucleotide bases joined weakly in the middle by hydrogen bonds.

Watson and Crick

What is a Gene?

• A gene is the basic unit of heredity passed from parent to offspring. Genes are pieces of DNA, and most genes contain the information for making a specific protein or coding for a particular trait.

• Humans are estimated to have 40,000-100,000 genes

A Mode of Inheritance: the missing component of Darwin’s Idea

Pangenesis? Acquired traits?

Pangenesis and the Blending of Bloods: An early hypothesis of how traits are passed from

one generation to the next. Pre-Mendel, this was a popular idea, and Darwin even considered it a

possibility!

This idea depends on the true mixing of characteristics from each parent.

Gregor Mendel(1822- 1884)

• A Moravian monk who discovered the principles of particulate inheritance through carefully conducted plant breeding experiments. His work still stands today.

Particulate Inheritance

Mendel’s work showed inheritance to bePARTICULATE—no mixing or blending ofthe traits, but a COMBINATION OF EFFECTS!

Mendel’s breeding experiments• Mendel created “pure

breeding” strains for various traits.

• He noted that in 1st

generation (F1) cross of these strains, certain (recessive) traits disappeared.

• However, in the F2 crosses they reappeared.

• This was strong evidence against blending inheritance.

The ¾ to ¼ ratio in Mendelian inheritance

Aa

A

A a

aaa

AaAA

A Punnett Square shows how the traitsare inherited (A=dominant trait; a=recessive trait).

Mendel’s Principle of Segregation

• Traits exist in discrete (or particulate) units; we call these alleles.

• An offspring gets one allele from each parent for a particular trait.

• His experiment also revealed the presence of dominant and recessive traits (see next slide for explanation).

Mendel Found Different Expressions of Genes…

• Dominant: alleles that are always expressed. Example: TT or Tt—in Mendel’s work, the tall trait is visible as long as the allele T is present.

• Recessive: alleles that are expressed when a dominant allele is NOT present. Example: tt—in Mendel’s work, short plants were only present when they had the recessive genotype.

• Codominant: when both alleles are expressed. Example: AB blood—the individual has both A and B types in their blood.

Tall and short pea plants

Mendel’s Principle of Independent Assortment

• In a situation where two traits are examined, and when each trait exists on a different chromosome pair…

• The traits will act INDEPENDENTLY because the chromosomes are inherited independently of one another.

Summary…

• So, from Mendel’s work, you should be getting the idea that:– He created our modern ideas of heredity,

though we did not know about them until after AD 1900.

– There are TWO principles that explain the particulars of heredity.

– We can predict the traits of offspring with a Punnett Square.

And now…a survey of Mendeliantraits

• Have some fun checking out the variety of traits that are inherited in Mendelian fashion (this means it is one gene that controls the trait—one mistake, and you have it!) ☺

• These are separated into traits that exist in DOMINANT form and RECESSIVE form. Remember, just because the trait is Dominant, does not make it MORE frequent in a population!

• Which traits do you have?

Some traits with dominant inheritance

Bent Little finger

Mid-digital hair Sort Big Toe

Darwin’s tubercle

• Bent little finger• Short big toe• Darwin’s

tubercle• Mid-digital hair

Brachydactyly

• Refers to shortening of the fingers and toes of the hands and feet.

• There are many different forms of this condition.

• Dominant inheritance

Dominant Inheritance of Pedigree of brachydactyly

Digital Depression

• Ring fingers longer than middle fingers have been associated with depression

• The prenatal development of fingers and gonads is controlled by the same gene.

• In men, relatively long fingers seem to be linked to low fetal testosterone levels.

• Robert Lewis Stephenson was said to be moody. This painting suggests that he had a long ring finger…a genetic link?

White Forelock

• A trait with dominant inheritance

• Sometimes associated with defective hearing and other problems

• Notice that dominant traits such as this don’t skip generations

Achondroplastic Dwarfism• A dominant trait.• If two achondroplastic

people marry, about 2/3 of their children dwarf and 1/3 of normal height.

• There is also a higher than normal frequency of spontaneous abortions and stillbirths, so being homozygous for the trait is probably incompatible with life.

Some traits with recessive inheritance

Counter clockwise whorl

Hitchhiker’s thumb

• Hitchhiker’s Thumb• Counter clockwise hair whorl• Alopecia• Albinism

Alopecia Albinism

Alopecia• A recessive trait.• The hair of people with

the condition falls out, but otherwise they are normal.

Pedigree of Alopecia universalis

Tay-Sachs Disease• Tay-Sachs disease caused by a recessive allele

that results in the absence hexosaminidase, an enzyme necessary for the breakdowns of a lipid called GM2 ganglioside accumulates abnormally in nerve cells.

• This causes progressive destruction of the central nervous system of afflicted children who usually die before they are 5 years old.

• Population Differences in Prevalence– People of eastern European (Ashkenazi)

Jewish descent. About one in every 27 American Jews is a carrier.

– Some French Canadians and Cajuns also have a high frequency.

– The U.S. population in general population and Sephardic Jews have a carrier frequency of 1 in 250.

This EM photo shows numerous membrane bound myelin figures in a patient with Tay-Sachs

Albinism• The most common form is

an autosomal recessive disorder.

• Afflicted individuals lack the enzyme (tyrosinase) necessary for producing the pigment melanin.

• characterized by absence of pigment in hair, skin, and eyes.

• Eye problems and sun sensitivity are common features.

Sickle Cell Anemia

• Normal red blood cells are fat and donut shaped—they carry lots of oxygen to our organs.

• People heterozygous individuals have some normal and some sickled (see photo below right) red blood cells.

• People homozygous for the condition have severely sickle-shaped red blood cells that cannot carry oxygen efficiently.

• We will learn more about the variation in SSA around the world in the upcoming weeks.

There are OTHER types of traits!

• Mendelian traits are controlled by a single gene, so are easily observed and studied.

• OTHER traits include:– Sex-linked traits (found on the sex

chromosomes)– Polygenic traits (controlled by multiple genes)

• Here are some examples of non-Mendeliantraits…

Sex-Linked Traits

X

X Y

X

XX XY

XY

XXXYXX

MaleOffspring

FemaleOffspring

FemaleParent

Sex Chromosomes

x

Y

• Inheritance involving genes on the X and Y chromosomes.• Recessive traits on the the X chromosome of females are

expressed in males (when there is no allele for the trait on the Y chromosome).

• Traits on the Y chromosome are only expressed in males (since females have two Xs!).

MaleParent

Sex-Linked Traits: Color Blindness

• Red-green color blindness is a sex-linked trait carried on the X chromosome.

• Color receptors in the eyes (cones) are abnormal in affected individuals

Normal Color Blind

Can you see the square?Retina showing rods and cones

Sex-Linked Traits: Hairy Ears

• Probably a Y-linked trait as females do not exhibit the trait.

• Found in high frequency in some endogamous human populations (who marry within local groups).

Sex-Linked Traits: Hemophilia

• Females (circles on the diagram) can carry the allele; males (squares on the diagram) are affected with the disease).

Sex-Linked Traits: Male Pattern Baldness

• Early baldness of the ordinary type has been thought to be autosomal dominant in males and to be autosomal recessive in females who transmit the trait if heterozygous but are bald only if homozygous.

• The transmission through many successive generations, as in the descendants of President John Adams, suggests the operation of a single major gene.

Four generations of male pattern baldness in the Adams family

Polygenic Inheritance

• Most traits are controlled by more than one gene, (i.e., polygenic).

• For some polygenic traits, a few different alleles can produce a large number of different phenotypes (appearance).

• The example of skin color at the left, illustrates this.

Polygenic Inheritance: Eye Color

• Despite what we all learned in high school biology, eye color is likely to be a polygenic trait. The early view that blue is a simple recessive has been repeatedly shown to be wrong by observation of brown-eyed offspring of 2 blue-eyed parents.

• Blue-eyed offspring from 2 brown-eyed parents is a more frequent finding.

• At least two separate genes, each with two incompletely dominant alleles, govern human eye color.

• A man and a woman, each heterozygous for both genes, could have children with five different eye colors, ranging from light blue (no dominant alleles) through light brown (two dominants) to almost black (all four alleles dominant).