- Slides: 77
Theoretical Genetics Mendelian Genetics
• 1865 - Austrian monk named Gregor Mendel published the results of his experiments of the inheritance of characteristics in pea plants. • DNA would not be discovered for another 100 years. • The characteristic that were inherited were referred to as “factors” since the term “gene” did not exist. • Mendel’s Principles of Heredity formed the foundation of Modern Genetics.
“Father of Genetics” Particulate Hypothesis of Inheritance Parents pass on to their offspring separate and distinct factors (today called genes) that are responsible for inherited traits.
Monohybrid Crosses of Traits
• P (parent) Generation- Plants are self pollinated for several generations to form pure or TRUE BREEDING off spring. – All of the off-spring will display the ONE form of the trait that the parent has. • Example) All purple flowers or all white flowers. • HOMOZYGOUS- Having TWO identical alleles for gene. (PP or pp)
• Mendel artificially cross pollinated two Pgenerations that had either form of a trait. (Example- pure purple with pure white)
• F 1 Generation- the cross of the P generation resulted in ALL of the offspring expressing ONE of the parent generation traits. • Example) All of the off-spring were purple. » Purple- DOMINANT ALLELE- trait that is always expressed when it is present. » White- RECESSIVE ALLELE- trait that is only expressed when it is homozygous. Allele is masked by the dominant allele.
HETEROZYGOUS (HYBRID)- Having two different alleles of a gene. (Pp) PP Pp Pp pp Pp Pp
• F 2 generation- Mendel allowed the F 1 generation to self pollinate. • The recessive trait (white) reappeared in a 3: 1 with the dominant trait (purple).
PHENOTYPE- The PHYSICAL characteristic or traits of an organism GENOTYPE- the GENETIC make up of a trait representing the pair of alleles.
Expression of Traits- at least 2 alleles needed. – 1. Phenotype- physical appearance of traits. » Tall, short – 2. Genotype- genetic make up of traits. – a. HOMOZYGOUS- alleles are the same » TT- Dominant -tall » tt- Recessive - short – b. HETEROZYGOUS- alleles are different » Tt- Tall * Dominant allele is expressed the recessive is masked.
Mendel’s Laws of Inheritance Law of Segregation Alleles separate during gamete formation and act independently. (Only one trait is passed to each gamete. ) • a. Alleles: different forms of the same gene. • b. An individual has two alleles for every trait. (Homologous Chromosomes) • c. One allele comes from each parent.
Law of Independent Assortment – When gametes are formed the separation of one pair of alleles between the daughter cells is INDEPENDENT of the separation of another pair of alleles. – The inheritance of one trait does not influence the inheritance of another trait on a different chromosome or of genes that are far apart on the same chromosome. Example) Seed Shape and Seed Color are independent of each other.
9: 3: 3: 1
• Why INDEPENDENT ASSORTMENT OCCURES: – CROSSING OVER during PROPHASE I – How the homologous chromosomes line up on the “equator” during METAPHASE I (Random Orientation) allows for random distribution of the alleles.
• Crossing Over in Prophase – Non-sister chromatids exchange genetic material or segments of DNA. • The maternal chromosome can end up with a segment from the paternal chromosome. RECOMBINANTS now have a different combination of alleles then the original coding.
– Two genes that are found on the same chromosome are said to be LINKED to each other. • Linked genes are usually passed on to the next generation together. (Goes against Mendel’s Law of Independent Assortment) • Groups of genes that are inherited together on the same chromosome are considered members of a LINKAGE GROUP. (A and B are a LINKAGE GROUP)
• The genes A and B are a linkage group. If this genotype (Aa. Bb) was crossed with itself would produce a 3: 1 ratio not a 9: 3: 3: 1 • This show linked genes are USUALLY inherited together.
BUT –Crossing over exchanges lengths of DNA. –In doing so the chromosomes of a homologous pair exchange their alleles allowing for variation in LINKED GENES. (RECOMBINANT gametes produced)
–This allows for the RECOMBINATION of alleles (a. B and Ab) –The LINKED GENES are AB and ab which are more likely to be inherited together. (Found in higher frequencies)
Crossing over results in genetic variation of the daughter cells.
• UNLINKED genes (not on the same chromosome) are randomly assorted during METAPHASE I and II.
Example of Di. Hybrid Cross Between Two Linked Genes Sweet Pea Crosses • Allele Key: • Flower color: Purple (P) and Red (p) • Pollen grain shape: Long (L) and short (l) • A cross was made between a plants that where heterozygotes at both gene loci (Pp. Ll).
• When dealing with linked genes the linkage group is drawn on the horizontal with a line representing the chromosome. • EXAMPLE) Heterozygotes at both locations P L _____ X _____ p l
• Cross: • The grid shows the cross with the possible fertilizations in the boxes. • There four expected genotypes. • There are four recombinant genotypes but cannot be observed from phenotypes. • There are two recombinant phenotypes which would be actually observed during the experiment.
TEST CROSS: MONOHYBRID Unknown genotype of the Dominant is crossed with a Homozygous Recessive.
TEST CROSS Heterozygotes (Ss. Yy) and Homozygote dominants (SSYY) cannot be distinguish by observing their PHENOTYPES. Mendel however developed a test cross for suspected heterozygotes.
The suspected heterozygote is crossed with a homozygous recessive at both gene loci. (ssyy) LOCI- position of a gene on a chromosome.
Set up of Cross • Meiosis produces four gametes in the heterozygote which illustrates the random orientation of chromosomes in metaphase I • Meiosis only produces one type of gamete for the homozyote double recessive.
Offspring genotypes show four different types. The phenotype ration is 1: 1: 1: 1 • What if the “suspect” was Homozygous Dominant? What would the off spring look like?
Theoretical Genetics Patterns of Inheritance
Sex Chromosomes • The 23 rd pair of chromosomes are called the SEX CHROMOSOMES because they determine if the individual is a male or a female.
• AUTOSOMES- the other 22 pairs of HOMOLOGOUS CHROMOSOMES that code for the body. (purple)
– The X chromosome is longer then the Y and contains more genes. – The X and the Y are very different in size and shape.
• Female – Human females have two X chromosomes. (XX) – All of her eggs produced by meiosis will contain one X chromosomes. • Males – Human males have one X and one Y chromosome. (XY) – Half of his sperm will contain one X and the other half will contain one Y. Each fertilization event results in a 50% chance of a girl (XX) and a 50% chance of having a boy (XY).
• Since the Y chromosome is so much smaller then the X chromosome it has fewer loci and fewer genes then the X chromosome. • Sometimes the allele on the X chromosome has no other allele to pair up with on the Y chromosome.
Homologous and Non-Homologous regions of the Sex Chromosomes MALE • For the male sex chromosomes their are non-homologous region males in which there is only one allele per gene and that is inherited from the female on the X-chromosome • In the homologous region the male inherited two copies of an allele per gene. As per the normal situation.
FEMALE • On the female sex chromosomes all regions of the X chromosome are homologous. • There are two alleles per gene as with all other genes on all other chromosomes This difference in x and y chromosomes plays a large role in determining rates of genetic inherited defects
Sex-Linked Traits • A sex-linked trait must have its locus on a sex chromosome. • Genetic traits which show sex linkage often affect one gender more then the other. – Ex) X- linked in the non-homologous section of a chromosome. - Most frequent in MALES • Baldness • Hemophilia • Colorblindness
• Color Blindness- the inability to distinguish between certain colors, often green and red. – X- linked acts as a recessive – XB is the NORMAL (dominant) allele – Xb is the affected allele ) • NORMAL MALE- XBY • NORMAL FEMALE- XBXB Why is there no such thing as a MALE CARRIER? • COLOR BLIND MALE- Xb Y (only need to inherit one affected allele to show the because there is nothing to dominate over it on the Y) • COLOR BLIND FEMALE- Xb Xb (must inherit both affected allele to show the trait ) • COLOR BLIND CARRIER- XBXb (Does not show trait but carries an allele to be passed onto offspring)
• Hemophilia- blood clotting disorder. Clotting proteins are not made. • NORMAL MALE- XHY • NORMAL FEMALE- XHXH • HEMOPHILIAC MALE- Xh Y (only need to inherit one affected allele to show the because there is nothing to dominate over it on the Y) • HEMOPHILIAC FEMALE- Xh Xh (must inherit both affected allele to show the trait ) • HEMOPHILIAC CARRIER- XHXh (Does not show trait but carries an allele to be passed onto offspring)
• CARRIERS– Sex- linked recessive alleles are rare in most populations. – Rare to find a colorblind or hemophiliac female because they must inherit two affected alleles for the trait. • Usually the normal gene dominates over the recessive in a carrier. – Males can not be carriers because they only need to inherit one of the affected allele because there is no normal gene to dominate over it.
• Some sex linked traits can be DOMINANTLY inherited (homologous section of chromosomes) or found on the Y CHROMOSOME (ONLY found in males).
INCOMPLETE DOMINANCE • Cross between 2 different phenotypes to produce a third phenotype, which is an intermediate or BLEND of the parents traits
• CODOMINANT ALLELES Two Dominant alleles are expressed at the same time. Both traits are expressed.
WW RR RW R RR RW WW
Sickle Cell Trait – makes both normal and sickle shaped RBC
Phenotypes Sickle cell trait Genotypes Hb. NHb. S Gametes Offspring Proportions Hb. N x Sickle cell trait Hb. NHb. S Hb. NHb. SHb. S Normal 25% Hb. S Sickle cell trait Sickle cell anaemia 50% 25%
• MULTIPLE ALLELES- genes with three or more alleles. (an individual only inherits two of the possible alleles) BLOOD TYPING
4. 3. 4 ABO Blood Groups • An example of multiple alleles is blood groups with 3 alleles • IAIB show BOTH A and B - CODOMINANT
Blood Type Test Cross
In rabbits, coat color is controlled by multiple alleles. Full color (C), white (c), light-gray or chinchilla (cch) and white with black points or a Himalayan (ch). Full color is dominant to all the other alleles. Chinchilla is dominant to Himalayan and white.
• Polygenic Inheritance involves two or more genes that influence the expression of one trait. – There are more alleles interacting allowing for a greater amount of variation. – Many human traits are inherited this way showing many different varieties. (CONTINUOUS VARIATION) • Examples) Eye color, skin color, height
• Continuous Variation shows a bell shaped curve in its frequencies showing all of the possible phenotypes. • (DICONTINUOUS Variation the results show a broken transition from trait to trait. )
• Blood types are DISCONTINUOUS
• Skin Color- There is an endless variety of shades of skin color. – Skin color is influence by the genetics of melanin production and of the environment (tanning). • The darker the skin the more concentrated the melanin. • The darker the skin the more Dominant inherited alleles for melanin production.
Plant Height • Another example of how the height of a plant is controlled by a polygenic system. Here there are two genes each with two alleles that control height. Again cross two heterozygote's at each loci. Allele key: • M 1= Tall plant • M 2= Small plant • N 1=Tall plant • N 2=Small plant
Plant Height 0 tall alleles 1 tall allele 2 tall alleles 3 tall alleles 4 tall alleles
PEDIGREE CHARTS are diagrams that are constructed to show biological relationships and how a trait is passed on from one generation to the next.
• Can be used to trace a DOMINANT INHERITED TRAIT, RECESSIVE INHERITED TRAIT or a SEX LINKED TRAIT. Hemophilia in the Royal Family (note males affected and female carriers)
Pedigree Chart • • 1. White circle : Normal female 2. White Square: Normal male 3. Black Circle: affected female 7. Black square: affected male (1) and (2). . Normal Parents (3) affected female offspring (4)normal male off spring