-628650-619125GENETICS and EVOLUTION
The cell Cycles
Cells in humans undergoes one of two types of cell divisions
Mitosis is a type of cell division that produces genetically identical cells. Important for growth, repair and asexual reproduction
Stages of Mitosis
Resting stage of cell.
chromosomes condense and become visible.
centrioles move to opposite poles of cell
phase ends with the breakdown of the nuclear membrane.
spindle fibers connect centrioles to chromosomes
chromosomes align along equator of cell and attaches to a spindle fibre by its centromere.
Chromatids are pulled toward opposite poles of cell by spindles.
Telophase and cytokinesis
Nuclear membrane reform around chromatids at each pole.
Cell membrane cleaves to form 2 daughter cells that are genetically identical to parent and to each other.
Unlike Mitosis, Meiosis occurs only in sex cells (gametes)
Meiosis is a type of cell division that produces genetically unidentical cells.
Stages of Meiosis
Same as mitosis.
Meiosis in egg and sperm production
Animal e.g. Plant e.g.
-400050226696 Sperm Egg
Importance of Meiosis
Daughter cells contain half the number of chromosomes as the parent cell.
Different sets of genes in daughter cells; create variation between each other and parent cell.
Meiosis occurs during formation of gametes, as basis of sexual reproduction.
Important to halve chromosome number (haploid) before fertilization which reinstates diploid number.
GENETICS – branch of biology that deals with heredity and variation of organisms.
Chromosomes carry the hereditary information (genes)
Chromosomes (and genes) occur in pairs called Homologous Chromosomes ( and alleles)
New combinations of genes occur in sexual reproduction
Fertilization from two parents
Gregor Mendel (father of Genetics)
2762250231775Mendel was the first biologist to use Mathematics – to explain his results quantitatively.
The concept of genes
That genes occur in pairs
That one gene of each pair ispresent in the gametes
Genetics terms you need to know:
Gene – a unit of heredity; a section of DNA sequence encoding a single protein
Alleles – two genes that occupy the same position on homologous chromosomes and that cover the same trait (like ‘flavors’ of a trait).
Locus – a fixed location on a strand of DNA where a gene or one of its alleles is located.
Homozygous – having identical genes (one from each parent) for a particular characteristic.
Homozygous dominant- BB
Homozygous recessive – bb
Heterozygous – having two different genes for a particular characteristic (Bb).
Dominant – the allele of a gene that masks or suppresses the expression of an alternate allele; the trait appears in the heterozygous condition.
Recessive – an allele that is masked by a dominant allele; does not appear in the heterozygous condition, only in homozygous.
Genotype – the genetic makeup of an organisms
Phenotype – the physical appearance of an organism (Genotype + environment)
Monohybrid cross: a genetic cross involving a single pair of genes (one trait); parents differ by a single trait.
P = Parental generation
F1 = First filial generation; offspring from a genetic cross.
F2 = Second filial generation of a genetic cross
Environmental variation: conditions caused by the conditions of an organism’s environment; they are not inherited, e.g. flower colour in hydrangea plants (continuous variation).
Genetic variation: variations due to genes; it can be inherited, e.g. hair colour in humans (discontinuous variation).
Parents differ by a single trait.
Crossing two pea plants that differ in stem size, one tall one short
T = allele for Tall
t = allele for dwarf
TT = homozygous tall plant
t t = homozygous dwarf plant
T T t tMonohybrid cross for stem length:
P = parentalstrue breeding,
304800139702057400299720T T t t(tall) (dwarf)
T t(all tall plants)
431482519050F1 generation is heterozygous:
A useful tool to do genetic crosses
For a monohybrid cross, you need a square divided by four….
Looks like a window
We use the
to predict the
genotypes and phenotypes of
Using a Punnett Square
STEPS: 1. determine the genotypes of the parent organisms 2. write down your “cross” (mating) 3. draw a p-square e.g. Parent genotypes:
TT and t tCross
T T t t4. “split” the letters of the genotype for each parent ; put them “outside” the p-square
5. determine the possible genotypes of the offspring by filling in the p-square
T T6. summarize results (genotypes ; phenotypes of offspring)
T tT tT t T tT T t t
100% T t
100% Tall plants
Monohybrid cross: F2 generation
35445702457451809750236855If you let the F1 generation self-fertilize, the next monohybrid cross would be:
T t T t(tall) (tall)
Phenotypic ratio= 3:1
1 TT= Tall
2 Tt = Tall
1 tt = dwarf
Genotypic ratio= 1:2:1
T TT t T t t tSecret of the Punnett Square
Key to the Punnett Square:
Determine the gametes of each parent…
T T t tHow? By “splitting” the genotypes of each parent:
If this is your cross
The gametes are:
Once you have the gametes…show the possible fusion during fertilization. e.g.
T tT tT t T tT
T tT tT t T tFor example, flower color:
P = purple (dominant)
p = white (recessive)
2676525287020Another example: Flower color
T tT tT t T tIf you cross a homozygous Purple (PP) with a homozygous white (pp):
P P x p p
ALL PURPLE (Pp)
Cross the F1 generation:P p x P p
1 PP: 2 Pp: 1 pp
3 Purple: 1 White
1. Principle of Dominance:
One allele masked another, one allele was dominant over the other in the F1 generation.
When you have an individual with an unknown genotype, you do a test cross.
Test cross: Cross with a homozygous recessive individual.
For example, a plant with purple flowers can either be PP or Pp… therefore, you cross the plant with a pp (white flowers, homozygous recessive)
165735088900-635 P ? pp
If you get all 100% purple flowers, then the unknown parent was PP…
If you get 50% white, 50% purple flowers, then the unknown parent was Pp…
Test cross – designed to reveal the genotype of an organism
403860046355Beyond Mendelian Genetics: Incomplete Dominance
Mendel was lucky!
Traits he chose in the pea plant showed up very clearly…
One allele was dominant over another, so phenotypes were easy to recognize.
But sometimes phenotypes are not very obvious…
right-391160Incomplete (partial) Dominance
Snapdragon flowers come in many colors.
If you cross a red snapdragon (RR) with a white snapdragon (WW)
You get PINK flowers (RW)!
Genes show incomplete dominance when the heterozygous phenotype is intermediate.
When F1 generation (all pink flowers) is self-pollinated, the F2 generation is a mixture of red, pink, and white flowers in the ratio of 1:2:1 respectively.
R RR W
R W WW
What happens if you cross a pink with a white?
A pink with a red?
Two alleles affect the phenotype in separate and distinguishable ways.
Neither allele can mask the other and both are expressed in the offspring independent of each other (therefore not in an “intermediate” form).
Example: red flowers that are crossed with white flowers that yield red and white flowers.
In cattle, roan coat color (mixed red and white hairs) occurs in the heterozygous (Rr) offspring of red (RR) and white (rr) homozygotes. When two roan cattle are crossed, the phenotypes of the progeny are found to be in the ratio of 1 red:2 roan:1 white. Which of the following crosses could produce the highest percentage of roan cattle?
A) roan x roan
B) red x white
C) white x roan
D) red x roan
E) All of the above crosses would give the same percentage of roan.
Many genes have more than two alleles in the population
Ex. three alleles for ABO blood type in humans
IA, IB, i
O – Universal Donor
AB – Universal Acceptor
Environmental Impact on Phenotype
pH of the soil will change the color of hydrangea flowers from blue to pink
Genetic crosses can be shown as pedigree charts
Family pedigrees are used to determine patterns of inheritance and individual genotypes
38100281305In the chart below; circles are female while squares are male.
In sperm, half have 22 chromosomes + X and half have 22 + Y. all eggs contain 22 + X.
If X-sperm from male and egg fuses during fertilization offspring will be female. However, if Y-sperm from male fuses with egg instead during fertilization offspring will become male.
Sex-linked disorders affect mostly males
Most sex-linked human disorders are due to recessive alleles
Ex: hemophilia, red-green color blindness
These traits appear mostly in males. Why?
In the sex chromosomes, the X is longer than the Y. this means alleles in the unmatched area of X can show themselves in a male, even though they are recessive, therefore:
If a male receives a single X-linked recessive allele from his mother, he will have the disorder; while a female has to receive the allele from both parents to be affected.
Characteristics inherited like this are called sex-linked.
Pedigree Chart: Inheritance Pattern for an X-linked Recessive Disease
Mutations are changes in the chromosomes or genes which leads to genetic variation. They can occur naturally as a result of radiation in the environment or genetic mistakes; are artificially in which they are caused by chemicals (mutagens) or x-rays.
Most mutations usually involve recessive alleles e.g.
A few however are caused by dominant alleles
Examples: achondroplasia, Huntington’s disease
center-600075Chapter 2: Natural Selection and Variation
Individuals of the same species show some differences in characteristics.
Humans have many variations from each other even when they belong to the same family. Eg. difference in height, hair type etc.
There are 2 basic kind of variation.
Discontinuous variation – fits into a definite category. Eg human blood groups, tongue rolling, free or attach earlobes etc. (control by only a single pair of genes – unaffected by environmental influence).
Continuous variation – characteristics varies between 2 extremes. It generally occurs where the environment has an effect on the characteristics and is usually controlled by multiple genes – eg human height, leaf size etc. continuous variation when plated on a graph forms a bell curve with the greatest distribution being in the middle.
A normal distribution curve
This is a graph that shows the number of people of different heights eg student’s height in a class.
Individuals with characteristics that give them an advantage over others are more likely to survive and reproduce. If these characteristics are cause by gene, then these characteristics will also appear in their offspring. Individuals without these features are more likely to die before they can reproduce. This is called natural selection.
Examples include: – peppered moths and antibiotic – resistant bacterial.
Equal number of dark and light peppered moths were collected and marked with a spot of paint on the underside.
Equal number of each type was released into a polluted (dark trees) wood and an unpolluted (light trees) wood.
After a few days flying moths were recaptured using a light trap.
Most recaptured flying moth from polluted wood was dark, suggesting light one has been eaten by birds. While from unpolluted woods, more light ones had survived.
Antibiotic resistance Bacteria
In a population of bacteria, not everyone is alike. By chance, one may have a gene that makes it resistant to an antibiotic.
Antibiotic is added which kills non-resistant bacteria.
The resistant one multiplies forming a population of resistant bacteria.
New species are formed when a population is isolated from another, so that it develops along its own lines. Eventually, it becomes unable to interbreed with members of the original species; and has therefore become a new species.
A population of beetles lives in a moist habitat, with plenty of vegetation.
A mountain range emerges dividing population in 2 and altering climate on one side of the mountain.
The Isolated population on the dryer side of mountain ridge gradually adapted to its new conditions.
When mountain barrier eroded sufficiently, the populations can meet once again, but are now so different they can no longer interbred.
Humans used artificial selection and breeding programs to produce populations of organisms with features that they find useful. Crops, plants, and some farm and domestic animals have been bred in this way eg, the Jamaican hope, most host cats and dogs etc.
Problem: What is a major danger of artificial selection?
Ans: The major danger of artificial selection is that traits that lower the fitness of a species can be increased in frequency in the population.
Problem: How might artificial selection be used to increase the yield of food crops such as corn?
Ans: Farmers can pick the plants that have produce more or larger ears of corn and allow only these plants to breed to create the next generation.
Problem: How is artificial selection different from natural selection?
Ans: Natural selection selects for or against traits based on their effect on the fitness of the organism. In artificial selection, traits are selected for or against based on human preference.
center-600075Chapter 2: Genetic Engineering
differences More recently, genetic engineering has been used to take genes from one organism and introduce (place) them into a different species. E.g. Include the introduction of insulin genes from human into bacteria which can be collected and used (used to mass produce increase yield to use in treatment therapy for diabetics.) and pest resistance genes into crop plants.
Diseases such as cystic fibrosis might also be treated by putting normal genes into an affected person.
Genetic engineering has the potential to produce great benefits, but there are many ethical, moral, social and ecological issues to be considered. E.g.
Social/economic: who should benefit, as costs are very high? Which crops should be changed and who does this help?
Ethical: is it right to change the genetics of organisms, and even more so, humans? Should work be limited to body cells, and not try to affect eggs and sperm cells?
Ecological: might the genes, for example put into genetically modified crops for good reasons, escape into the environment.
Supporters of genetic engineering in agriculture point to a multitude of potential benefits of engineered crops, including increased yield, drought tolerance, reduced pesticide use, more efficient use of fertilizers, and ability to produce drugs or other useful chemicals. However, Scientific analysis shows that actual benefits have often fallen far short of expectations.
While the risks of genetic engineering are often exaggerated or misrepresented, GE crops do have the potential to cause a variety of health problems and environmental impacts. For instance, they may spread undesirable traits to weeds and non-GE crops, produce new allergens and toxins, or harm animals that consume them.
Stages of genetic engineering
A basic technique used is the genetic engineering of bacteria. It can be broken into the following key stages:
Selection of characteristics.Identifying the gene from amongst all the others in the DNA of the donor organism.
Isolation of the gene.Obtaining a copy of the required gene from the DNA of the donor organism and placing it in a vector (Plasmid – circular DNA found in cytoplasm of bacteria’s).(A vector in biology refers to an organism that acts as a vehicle to transfer genetic material from a donor organism to a target cell in a recipient organism.)
InsertionUse the vector to introduce the gene into the host cell.
ReplicationAllow the host cell to multiply to make multiple clones of the genes.
Recombinant DNA Technology
-542925290195Production of insulin using E. coli bacteria;