Mendel’s Genetics

Submitted by: Deborah G. Daquioag

Mendelian Genetics Definitions• Allele - one alternative form of a

given allelic pair; tall and dwarf are the alleles for the height of a pea plant; more than two alleles can exist for any specific gene, but only two of them will be found within any individual

• Homozygote - an individual which contains only one allele at the allelic pair; for example DD is homozygous dominant and dd is homozygous recessive; pure lines are homozygous for the gene of interest

• Allelic pair - the combination of two alleles which comprise the gene pair

• Heterozygote - an individual which contains one of each member of the gene pair; for example the Dd heterozygote

• Phenotype - literally means "the form that is shown"; it is the outward, physical appearance of a particular trait

• Genotype - the specific allelic combination for a certain gene or set of genes

• Monohybrid cross - a cross between parents that differ at a single gene pair (usually AA x aa)

Mendelian Genetics Definitions• Dominant - the allele that

expresses itself at the expense of an alternate allele; the phenotype that is expressed in the F1 generation from the cross of two pure lines

• Recessive - an allele whose expression is suppressed in the presence of a dominant allele; the phenotype that disappears in the F1 generation from the cross of two pure lines and reappears in the F2 generation

• Dihybrid cross - a cross between two parents that differ by two pairs of alleles (AABB x aabb)

• Dihybrid- an individual heterozygous for two pairs of alleles (AaBb)

Mendelian Genetics Definitions• Backcross - the cross of an

F1 hybrid to one of the homozygous parents; for pea plant height the cross would be Dd x DD or Dd x dd; most often, though a backcross is a cross to a fully recessive parent

• Testcross - the cross of any individual to a homozygous recessive parent; used to determine if the individual is homozygous dominant or heterozygous

• Monohybrid - the offspring of two parents that are homozygous for alternate alleles of a gene pair

• Dominance - the ability of one allele to express its phenotype at the expense of an alternate allele; the major form of interaction between alleles; generally the dominant allele will make a gene product that the recessive can not; therefore the dominant allele will express itself whenever it is present

Mendel’s Genetics• By the 1890's, the invention of

better microscopes allowed biologists to discover the basic facts of cell division and sexual reproduction.

• The focus of genetics research then shifted to understanding what really happens in the transmission of hereditary traits from parents to children.

• A number of hypotheses were suggested to explain heredity, but Gregor Mendel , a little known Central European monk, was the only one who got it more or less right.

Mendel’s Genetics• His ideas had been published

in 1866 but largely went unrecognized until 1900, which was long after his death.

• His early adult life was spent in relative obscurity doing basic genetics research and teaching high school mathematics, physics, and Greek in Brno (now in the Czech Republic).

• In his later years, he became the abbot of his monastery and put aside his scientific work.

Mendel’s Genetics• Mendel picked common

garden pea plants (Pisum sativum) for the focus of his research because they can be grown easily in large numbers and their reproduction can be manipulated

• Mendel discovered that certain traits show up in offspring without any blending of parent characteristics.

Mendel’s Genetics• The seven traits that are easily

recognized and only occur in one of two forms:

1) flower color is purple or white 2) flower position is axil or

terminal3) stem length is long or short4) seed shape is round or wrinkled5) seed color is yellow or green6) pod shape is inflated or

constricted7) pod color is yellow or green

Mendel’s Genetics• Pea plants have both male and

female reproductive organs. As a result, they can either self-pollinate themselves or cross-pollinate with another plant.

• In his experiments, Mendel was able to selectively cross-pollinate purebred plants with particular traits and observe the outcome over many generations.

• This was the basis for his conclusions about the nature of genetic inheritance.

Mendel’s Genetics• In cross-pollinating plants that either produce yellow

or green pea seeds, Mendel found that the first offspring generation (f1) always has yellow seeds.

• However, the following generation (f2) consistently has a 3:1 ratio of yellow to green.

(all heterozygous)

Mendel’s Genetics

• This 3:1 ratio occurs in later generations as well. Mendel realized that this was the key to understanding the basic mechanisms of inheritance.

Mendel’s Genetics• He came to three important

conclusions from these experimental results:

1) that the inheritance of each trait is determined by "units" or "factors" that are passed on to descendents unchanged

2) that an individual inherits one such unit from each parent for each trait

3) that a trait may not show up in an individual but can still be passed on to the next generation.

Mendel’s Genetics• With all of the seven pea plant

traits that Mendel examined, one form appeared dominant over the other, which is to say it masked the presence of the other allele.

• For example, when the genotype for pea seed color is YG (heterozygous), the phenotype is yellow. However, the dominant yellow allele does not alter the recessive green one in any way.

• Both alleles can be passed on to the next generation unchanged.

Mendel’s Genetics

• Mendel's observations from these experiments can be summarized in two principles:

1) the principle of segregation

2) the principle of independent assortment

Segregation

• for any particular trait, the pair of alleles of each parent separate and only one allele passes from each parent on to an offspring. Which allele in a parent's pair of alleles is inherited is a matter of chance.

• this segregation of alleles occurs during the process of sex cell formation (Segregation of alleles in the

production of sex cells)

Independent Assortment

• Independent assortment answers the question of what happens when two parent cells differ in two or more genes

• Mendel discovered independent assortment when he performed experiments called dihybrid crosses

Independent Assortment• alleles of different genes assort

independently from one another during gamete formation

• happens when genes are on different chromosomes

• does not hold true for genes that are on that same chromosome because if two genes are close enough together on the same chromosome then they may be linked in which case they stay together while crossing over. This phenomenon is called linkage

References: • http://www.monteweston.com/Biology/Jasm10.html• http://www.google.com.ph/search?

tbm=isch&hl=en&source=hp&biw=1366&bih=610&q=independent+assortment&gbv=2&oq=independent+a&aq=0&aqi=g10&aql=&gs_sm=1&gs_upl=1219l6097l0l7772l13l13l0l5l5l0l63l349l8l8l0

• http://www.google.com.ph/search?tbm=isch&hl=en&source=hp&biw=1366&bih=653&q=gregor+mendel&gbv=2&oq=gregor+men&aq=0&aqi=g10&aql=&gs_sm=1&gs_upl=645l3132l0l4935l10l10l0l2l2l0l97l476l8l8l0#hl=en&gbv=2&tbm=isch&sa=1&q=law+of+segregation&pbx=1&oq=law+og+se&aq=0sS&aqi=g-sS1&aql=&gs_sm=3&gs_upl=953646l957289l8l959174l13l13l2l0l0l0l184l990l7.4l11l0&bav=on.2,or.r_gc.r_pw.r_qf.,cf.osb&fp=3f24af890326a12a&biw=1366&bih=610

• http://anthro.palomar.edu/mendel/mendel_1.htm• http://www.ndsu.edu/pubweb/~mcclean/plsc431/mendel/mendel1.htm• http://www.ndsu.edu/pubweb/~mcclean/plsc431/mendel/mendel3.htm• http://www.google.com.ph/search?

tbm=isch&hl=en&source=hp&biw=1366&bih=653&q=gregor+mendel&gbv=2&oq=gregor+men&aq=0&aqi=g10&aql=&gs_sm=1&gs_upl=645l3132l0l4935l10l10l0l2l2l0l97l476l8l8l0#hl=en&gbv=2&biw=1366&bih=610&tbm=isch&sa=1&q=pea+&pbx=1&oq=pea+&aq=f&aqi=g10&aql=&gs_sm=3&gs_upl=2403l2403l15l2724l1l1l1l0l0l0l0l0ll0l0&bav=on.2,or.r_gc.r_pw.r_qf.,cf.osb&fp=3f24af890326a12a