Unit III: GENETICS

Classical (Mendelian) Genetics

Classical Genetics



Genetics

: This is the branch of biology that studies the ways in which hereditary information is passed on from parents to offspring. It includes principles in variation and inheritance.



Heredity

: this is the passing on of trait from parents to offspring.

The Blend Theory of Inheritance

 This is the early belief (before Mendel) that factors from the parents were blended in their offspring.  This theory was not able to explain the appearance or disappearance of distinct traits (dominant or recessive) from one generation to another.

Gregor Mendel and the Scientific Process

 Gregor Mendel used the scientific process to lay the foundation for the science of genetics.  Mendel used garden peas.  They were a good choice because they were easy to grow, matured quickly and showed distinct traits.

Mendel studied seven pairs of contrasting traits as follows:

Trait Dominant Recessive

Seed Shape Seed Color Seed Coat Color Pod Color Pod Shape Stem Length Flower Position

Round yellow Grayish brown Green Inflated Long Lateral Wrinkled Green White Yellow Wrinkled Short Terminal

Mendel’s Pea Plants

 Pea plants normally self pollinated because they contain the stigma and anther enclosed in the petals.  Mendel could easily cross fertilize by removing the stamens before they cross fertilize and dust the pollen from another anther onto the stigma.

Cross Pollination

 Mendel cross pollinated pure plants (

purebreds

) with contrasting traits.  These pure plants were called the parent or

P generation

.  In the offspring, Mendel observed that one trait would completely disappear.  For example, in a cross between tall and short plants, only tall plants would be present .

 The offspring are considered to be the 

first filial

or

F 1 generation

. The offspring of purebreds are known as are known as

hybrids

.

 Mendel allowed the hybrids of the F 1 generation to self pollinate.  This resulted in the F 2 generation. In this case, ¾ of the offspring were tall and ¼ were short.  This showed that the factor that contained the shortness had not disappeared.

 The “factors” described by Mendel were later called

genes

.

 The traits expressed in the F 1 generation are considered to be

dominant

because they mask another trait.  The hidden traits that appeared in the F 2 generation were considered to be

recessive

.

Unit Theory of Inheritance

 Mendel reasoned that for every trait there must be two governing factors (

unit characters

).  One of these factors came from the mother and the other factor came form the father during zygote formation.  These factors are separated during gamete formation.  This lead to the following of Mendel’s Laws:

Law of Dominance



Law of Dominance

: When an organism is hybrid for a pair of contrasting traits, only the dominant trait can be seen in the hybrid. A dominant trait is indicated by an upper case letter (ie. R) whereas a recessive trait is indicated by a lower case letter (ie. r)

Law of Segregation



Law of Segregation

: The idea that “factors” (

genes

) occur in pairs (

alleles

) and are separated from each other in gamete formation and recombined in fertilization.

 For more on Mendel and his experiments go to:  http://anthro.palomar.edu/mendel/mendel_1.htm

Law of Independent Assortment



Law of Independent Assortment

: genes for different trait are separated and distributed to gametes independently from each other.

  This law is not considered true today because many genes are found on homologous chromosomes and can not be separated. Genes found on the same chromosome are said to be linked.

http://www.sumanasinc.com/webcontent/ anisamples/majorsbiology/independentass ortment.html

Terms



Trait

: a characteristic determined by a gene ie. eye color.



Gene

: a distinct unit of hereditary material found in chromosomes.



Allele

: Two or more alternate forms of a gene for a trait.



Homozygous

: A genotype in which both genes of a pair are identical.



Heterozygous

:a genotype in which the gene pairs are different.



Co-dominant alleles

: Alleles that are simultaneously expressed in the heterozygous condition (AB blood type). Can be symbolized by upper case letters. 

Incomplete dominant alleles

: Alleles that are simultaneously expressed in an altered form – snap dragons hybrids are pink not white or red.



Genotype

: The genetic make-up of an organism.



Phenotype

: The physical traits an organism develops as a result of its genotype.



Punnett Square

: a chart used by geneticists to show the possible combinations of alleles in an offspring.

P E Question

 Which are alternate forms of genes?

 (A) alleles  (B) chromatids  (C) hybrids  (D) sub units

P E Question

     Which refers to the physical appearance of an organism?

(A) genetic drift (B) genetic heritage (C) genotype (D) phenotype

P E Question

     How did Mendel obtain the F1 generation of all hybrid tall pea plants?

(A) hybrid tall × hybrid tall (B) hybrid tall × pure tall (C) pure short × pure short (D) pure tall × pure short

P E Question

      See . 531… The Product Rule What is the probability of parents having the following three children: two boys in a row followed by a daughter?

(A) 1/16 (B) 1/8 (C) 1/4 (D) 1/2

Crosses



Monohybrid Crosses

 This is a chart to show the results of a cross between parents for a single trait. (2 x2) 

Dihybrid Crosses

 This is a chart to show the results of a cross between parents for two traits. (4 x4)

Homozygous Male with Homozygous Female

 A yellow round seed male is crosses with a green wrinkled seed female.

 Yellow- Round seed Male (YYRR)  Green – Wrinkled seed female (yyrr)

Test Cross

 A test cross is used to determine an unknown genotype that may either be homozygous dominant or heterozygous for a trait.  A individual of the unknown genotype is mated with an individual showing the contrasting recessive trait.

Example of a Test Cross:

 A breeder wishes to know if a pea plant is TT or Tt. He will cross the plant with a homozygous recessive plant (tt).  If the results show all tall plants than the unknown plant must be TT. If 50% are short, then the unknown plant must be Tt. Why? Attempt the crosses in the space below.

Co-dominance

 The condition in which both alleles of a gene are expressed. (ie. AB blood type) 

Example:

white hairs A cross between homozygous red shorthorn cattle and homozygous white shorthorn cattle result in heterozygous offspring with a roan coat. Roan is a mixture of red and C R C R x C W C W  C R C W

Incomplete Dominance

 Both alleles contribute to the phenotype of a heterozygous individual to produce a trait which is not exactly like either parent.



Example

: Red flowers (RR) combine with white flowers (R  R  ) to produce pink flowers (RR  ).

P E Question

     In horses, roan coats (red and white hairs) result from codominance. If two roan coat horses are crossed, what would be the expected phenotype ratios?

(A) all roan (B) ½rroan, ¼ red, ¼ white (C) ½ roan, ½ red (D) ½rroan, ½ white

P E Question

     Which type of inheritance causes variation in height and skin colour in humans?

(A) co-dominance (B) incomplete dominance (C) multiple alleles (D) polygenic inheritance

Polygenic Inheritance (Multiple Gene Inheritance)

 The traits studied by Mendel are controlled by the alleles of a single gene. Many traits of both plants and animals do not appear in just two contrasting forms. For example, humans are not just tall or short. Traits that vary between two extremes are controlled by the alleles of two or more different genes.

 When two or more independent genes affect a characteristic, it is called polygenic or multiple gene inheritance. Examples of traits controlled by polygenic inheritance include: eye color, skin color, height, facial features.

P E Question

     Eye colour for fruit flies is determined by genes X, Y and Z. Which is illustrated if X produces black eyes, Y produces brown eyes and Z produces magenta (purple) eyes?

(A) co-dominance (B) incomplete dominance (C) multiple alleles (D) polygenic inheritance

Multiple Alleles

 In this case, there are more than two alleles for a particular trait. In humans, there a three alleles that control blood type: A, B and O. A and B are equally dominant. O is recessive.

Genotype Blood Type

I A I A or I A i I B I B or I B i I A ii I B A B AB O

Example of Multiple alleles

 A woman with type A blood marries a man who has type B blood. They have five children, with type AB blood. What are the most probable parental genotypes?

P E Question

 Which are possible blood types for the parents of a child with AB blood?

Walter Sutton and Theodore Boveri

 1902 - studied phases of meiosis and realized that the behaviour of chromosomes during meiosis was similar to the behaviour of Mendel’s “factors.”  Noticed that chromosomes occur in pairs and that these separate during anaphase 1 of meiosis.

 Also noticed that chromosomes align independently along the cell’s equator.  Result is that each gamete receives one chromosome of each pair and that this one chromosome does not influence which other chromosomes will be found in that gamete.

Chromosome Theory of Inheritance

  These observations formed the basis for the

chromosome theory of inheritance.

It states : 1. Mendel’s factors , or genes are carried on chromosomes.

 2. It is the segregation and independent assortment of chromosomes during meiosis that accounts for the patterns of inheritance.

 * The chromosome theory can also account for patterns of inheritance that do not follow Mendel’s laws.

Thomas Morgan ( 1910)

 Investigated eye colour in fruit flies and produced a white- eyed male by crossing two red-eyed parents.

 This was similar to other monohybrid crosses.

 However , when he crossed a red eyed female offspring of the white eyed male with a normal red eyed male the results were :  All females - red eyes  Males - half had red eyes and half had white eyes.

Morgan …

    Morgan deduced that the gene for eye colour was located on the X chromosome.

This was the first time that a gene had been linked to a specific chromosome.

H

e also found that some genes do not follow the law of independent assortment because they tend to be inherited together.

For example : genes on the same chromosome cannot be separated. They are called

linked genes.

However , later in his studies he found that

sometimes

linked genes

do separate.

How can this happen ?

 Gene-chromosome theory :  1.

Genes exist at specific sites arranged in linear fashion along chromosomes.

 2. When pairs of homologous chromosomes separate during gamete formation, they form two gametes.

 3. Each gamete will contain a separate allele for each trait.

 4. During fertilization , chromosomes from one gamete will combine with another gamete.



Therefore : genes on chromosomes which are very close together will almost always be inherited together.



And : genes located far apart are more likely to be separated during a crossing over event.

 

The likelihood of crossing over increases with the distance between the two genes.

Morgan’s work indicates that Mendel’s law of

independent assortment can be restated as:

P E Question

     What is one of Morgan’s major contributions to the field of genetics?

(A) concept of codons (B) double helix (C) Law of Dominance (D) sex-linked traits

P E Question

     What does the Law of Independent Assortment state?

(A) Gene pairs always sort in the same order.

(B) Gene pairs sort randomly and independently from each other.

(C) One allele is always dominant.

(D) Sister chromatids separate during gamete formation.

Independent Assortment Restated:

 If crossing over does not take place , genes that are located on different chromosomes will assort independently while genes that are located on the same chromosome will be inherited together.

Sex - Linked Inheritance

 Some traits are passed from one generation to the next depending on the sex of the parent carrying the trait.

 This is because the genes for these traits is carried on the sex chromosomes  The transmission of genes that are located on the sex chromosomes , X or Y , is called

sex-linked inheritance.

Y-marks the spot?

 A gene that is only on the X chromosome is called X-linked  A gene that is only on the Y chromosome is called Y-linked.

 This may be due to the smaller size of the Y chromosome.

Why are sex-linked defects more common in males than females ?

 Sex linked inheritance involves pairs of genes on the X chromosome.

 Note: In terms of gene expression , autosomal ( non-sex chromosomes) inheritance typically involves pairs of genes , with gender being irrelevant to gene expression.

 Most sex-linked traits are X-linked.Very few Y linked traits are known.

 Sex-linked inheritance involves pairs of genes on the X chromosome in the female, and a single X in the male. In this case , gender is important in gene expression and must be considered a part of the phenotype.

P E Question

     If a colour blind man and a woman who is a carrier of colour blindness have a son, what are the chances that their son will be colour blind?

(A) 25% (B) 50% (C) 75% (D) 100%

 Three women gave birth to children in the same hospital at the same time. The babies are mixed up in the nursery. The hospital checked the blood type of the parents and children and the following data was obtained:      Who could have produced Baby Bob?

(A) either the Olsens or Jones (B) either the Smiths or Jones (C) only the Olsens (D) only the Smiths