the experiments of gregor mendel genetics – the study of heredity mendel – studied ordinary pea...
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Chapter 11Introduction to Genetics
11.1 The Work of Gregor Mendel
The Experiments of Gregor MendelGenetics – the study of heredityMendel – studied ordinary pea plantsThe Role of Fertilization
Pea plants – self-pollinating – sperm cells fertilize egg cell within same flower
Fertilization – production of a new cellMendel’s pea plants were “true-breeding” – self-pollinating and
each successive generation would be the same. Trait – a specific characteristic, such as seed color or plant height, of an
individual.Mendel crossed “true-breeding” plants – caused one plant to
reproduce with another plant.Hybrid – the offspring of crosses between parents with different
traits.
Genes and AllelesGenes – an individual’s characteristics are determined
by factors, called genes, that are passed from one parental generation to the next.
Alleles – different forms of a gene Plant height – Tall plant, short plant
Dominant and Recessive Alleles Principle of Dominance – some alleles are dominant and
others are recessive.Dominant – genes that keep other genes from showing
their traits Ex: DD
Recessive – do not show their traits when dominant genes are present Ex: dd
a
F1 Generation – hybrids had traits of only one parentWhere did the recessive alleles go? Did they
disappear?Mendel allowed seven F1 hybrids to self-pollinate
F2 Generation – ¼ of F2 plant showed recessive allele
At some point allele for tallness separated from allele for shortness - segregation
Gametes – sex cellsDuring gamete formation, the alleles for each gene
segregate from each other, so that each gamete carries only one allele for each gene.
11.2 Applying Mendel's Principles
Probability and Punnett SquaresProbability – likelihood that a particular event
will happen.Flipping a coin
Homozygous – organisms that have two identical alleles for a gene – TT or tt
Heterozygous – organisms that have two different alleles for the same gene – Tt
Genotype – genetic makeup – TTPhenotype – physical traits – Tall plant
The Punnett SquareA way to show which genes can combine
when egg and sperm join
Independent AssortmentGenes for different traits can segregate
independently during the formation of gametes.
First with Third, First with Fourth!Second with Third, Second with Fourth!R r Y y x R r Y y
• Always keep the same letters together! RrYy
• Keep Offspring in same letter format as Parents!
Always keep the same letter first in dihybrid crosses, even if it is recessive!!
R’s then Y’s
RrYy
RRYY
Whatever the
genotype!!!!!
11.3 Other Patterns of Inheritance
Beyond Dominant and Recessive Alleles
Incomplete Dominance – one allele is not completely dominant over another.
Codominance – the phenotypes produced by both alleles are clearly expressedHusky eyes
Multiple Alleles – many genes exist in several forms and therefore are said to have multiple allelesRabbit coat, Human blood types
Polygenic Traits – Many traits produced by interaction of several genesPolygenic – “many genes”Human skin color – more than four different genes
Genes and the EnvironmentEnvironmental conditions can affect gene
expression and influence genetically determined traits.
11.4 Meiosis
Chromosome NumberDiploid Cells – most adult organisms contain
two complete sets of inherited chromosomes and two complete sets of genesHumans = 2N = 46
Haploid – the gametes of sexually reproducing organisms are haploid.Human sex cells = N = 23Human sex cells HAVE to be haploid because
you get half from your mom’s egg (23) and half from your dad’s sperm (23) = 46 chromosomes in your body cells!
Phases of Meiosis
Prophase I – replicated chromosome pairs with its corresponding homologous chromosomes.Tetrad – pairing which contains four
chromatids.Crossing over – homologous chromosomes
cross over, alleles exchanged. Chromatids of the homologous chromosomes cross
over one another. Then, the crossed sections of the chromatids – which contain alleles – are exchanged.
Produces new combinations of alleles in the cell.
Metaphase I – paired homologous chromosomes line up across center of cell.
Anaphase I – spindle fibers pull each homologous chromosome pair toward opposite ends of the cell.
Telophase I – Nuclear membrane forms around each cluster of chromosomes.Cytokinesis – forming two new cellsMetaphase I results in two cells, called
daughter cells
Prophase II – chromosomes, consisting of two chromatids, condense and become visible.Do not form tetrads, because homologous pairs were
already separated during meiosis I.Metaphase II – chromosomes line up down center
of each cell.Anaphase II – Paired chromatids separate.Telophase II and Cytokinesis – four daughter cells
contain a haploid number.
Haploid cells produced by meiosis II are the gametes that are so important to heredity.Male – spermFemale - egg
Zygote – egg after it is fertilized.Undergoes cell division by mitosis and forms
new organism.
Comparing Mitosis and Meiosis
Comparing Mitosis and MeiosisReplication and Separation of Genetic Material
Mitosis – when two sets of genetic material separate, each daughter cell receives one complete set of chromosomes.
Meiosis – homologous chromosomes line up and then move to separate daughter cells. Two alleles for each gene are segregated, and end up in different cells.
Changes in Chromosome NumberMitosis does not normally change the chromosome number of
the original cell.Meiosis reduces the chromosome number by half.
Number of Cell DivisionsMitosis results in the production of two genetically identical
diploid cells, whereas meiosis produces four genetically different haploid cells.
Gene Linkage and Gene MapsGene Linkage – Alleles of different genes tend to be inherited together
from one generation to the next when those genes are located on the same chromosome. Thomas Hunt Morgan – 1910 – identified 50 Drosophila genes that
appeared to be “linked” together in ways that seemed to violate the principle of independent assortment.
Reddish-orange eyes and miniature wings were almost always inherited together. Morgan’s Findings led to two conclusions:
1. Each chromosome is actually a group of linked genes2. Mendel’s principle of independent assortment still holds true. It is chromosomes,
however, that assort independently, not individual genes.
Gene Mapping – Alfred Sturtevant wondered if the frequency of crossing-over between genes during meiosis might be a clue to the genes’ locations.
Further apart – more likely crossing-over Closer together – crossovers should be rare By this reason, he could use the frequency of crossing-over between
genes to determine the distances from each other.