Mendelian Genetics, Meiosis and Evolution • Mendel’s Laws of Heredity (10.1) • Meiosis (10.2) • The Theory of Evolution (15)
Dec 20, 2014
Mendelian Genetics, Meiosis and Evolution
• Mendel’s Laws of Heredity (10.1)• Meiosis (10.2)
• The Theory of Evolution (15)
Answer these questions… What is heredity?
The passing of traits from parents to offspring What is genetics?
The branch of biology that studies heredity What is sexual reproduction?
Male sex cell + Female sex cell Fertilized cell Male gamete + Female gamete Zygote
What is the above process called? Fertilization
In a plant, what does a zygote develop into? A seed
What is pollination? The transfer of pollen grains from a male reproductive organ
to a female reproductive organ in a plant What is a hybrid?
The offspring of parents that have different forms of a trait
Gregor Mendel
Old thinking: Blending Traits from both parents
get blended and the offspring is a mixture of both
Cross-pollinated pea plants to study how traits are passed from parents to offspring
Mendel’s 3 Conclusions
The rule of unit factors Each organism has two factors that control each of
its traits Factors = genes (located on chromosomes) Genes exist in alternative forms
Alleles Example: A pea plant could have 2 alleles for purple,
2 alleles for white or 1 allele for purple and 1 allele for white
An organism’s two alleles are located on different copies of a chromosome (one from the female parent, one from the male)
Mendel’s 3 Conclusions The rule of dominance
Purple flower + White flower = Purple flower Only the purple trait is observed, therefore it is
dominant The white trait ‘disappeared’, therefore it is
recessive
The law of segregation Every individual has two alleles of each gene When gametes (sex cells) are produced each
gamete receives one of these alleles During fertilization, these gametes randomly pair
to produce four combinations of alleles
Answer these questions…
What are the alleles of the purple-flowered plant in the parental generation? 2 for purple
The white-flowered? 2 for white
The first gen. offspring? 1 for purple, 1 for
white
Phenotypes and Genotypes The purple parent has 2 purple alleles and
the purple offspring has 1 purple and 1 white allele
Therefore, two organisms can look alike, but have different underling allele combinations
The way an organism looks and behaves is called its phenotype
The allele combination an organism contains is called its genotype
Phenotypes and Genotypes
If an organisms alleles are the same for a trait (think purple parent) then the organism is homozygous for that trait
If an organisms alleles are different for a trait (think first gen. purple offspring) then the organism is heterozygous for that trait
Mendel’s conclusions allow us to… Predict the probability of the genotype of an
offspring when given the parents genotype Determine a phenotype based on genotype
Answer this question…
If two plants are crossed that have two different traits, will the two traits stay together or will they be inherited independently of each other?
The Law of Independent Assortment
Genes for different traits are inherited independently of each other
Answer these questions… Where are genes located?
Chromosomes How many chromosomes do humans have?
46 (23 pairs) How many chromosomes in each cell?
46 (23 pairs) Where do these chromosomes come from?
23 from mom, 23 from dad Which cells combine to form a zygote (and then
offspring)? Gametes (sex cells)
Therefore, how many chromosomes in a gamete? 23
Diploid and Haploid Cells A cell with pairs of each chromosome is called
a diploid cell (2n) A cell with one of each chromosome is called a
haploid cell (n)
The two chromosomes of each pair in a diploid cell are called homologous chromosomes Not identical Contain information for the same traits Can have different alleles
Typical Animal Life Cycle
Homologous Chromosomes (Homologs)
Chromosomes 1 & 2 are homologous chromosomes
Chromosomes 3 & 4 are homologous chromosomes
Chromosomes 1 & 3 came from the mother
Chromosomes 2 & 4 came from the father
Meiosis – Overview
Meiosis is a special type of cell division that occurs in sexually reproducing organisms Chromosome number reduced by half, enabling
sexual recombination to occur. Meiosis of diploid cells haploid daughter cells (which
may function as gametes) Gametes undergo fertilization, restoring the diploid
number of chromosomes in the zygote
Meiosis – Overview
Meiosis and fertilization introduce genetic variation in three ways: Crossing over between homologous
chromosomes at prophase I Independent assortment of homologous pairs
at metaphase I Each homologous pair can orient in either of two ways
at the plane of cell division Random chance fertilization between any one
female gamete with any other male gamete
Meiosis - Overview
Sexual reproduction in a population should decline in frequency relative to asexual reproduction Asexual – No males are needed, all individuals
can produce offspring Sexual – Only females can produce offspring,
therefore fewer are produced Sexual reproduction may exist because it
provides genetic variability that reduces susceptibility of a population to pathogen attack This is the role of sexual reproduction in
evolution
Meiosis
2 main stages Meiosis I
Prophase I, Metaphase I, Anaphase I, Telophase I
Meiosis II Prophase II, Metaphase
II, Anaphase II, Telophase II
Meiosis I – Prophase I The chromosomes condense and
become visible The centrioles form and move toward
the poles The nuclear membrane begins to
dissolve The homologs pair up, forming a
tetrad Each tetrad is comprised of four
chromotids - the two homologs, each with their sister chromatid
Homologous chromosomes will swap genetic material in a process known as crossing over Crossing over serves to increase genetic
diversity by creating four unique chromatids
Meiosis I – Metaphase I
Microtubules grow from the centrioles and attach to the centromeres
The tetrads line up
along the cell equator
Meiosis I – Anaphase I
Homologous chromosomes separate (note that the sister chromatids are still attached)
Cytokinesis begins
Meiosis I – Telophase I
The chromosomes may decondense (depends on species)
Cytokinesis reaches
completion, creating two haploid daughter cells
Meiosis II – Prophase II
Centrioles form and move toward the poles
The nuclear
membrane dissolves
Meiosis II – Metaphase II
Microtubules grow from the centrioles and attach to the centromeres
The sister chromatids
line up along the cell equator
Meiosis II – Anaphase II
The centromeres break and sister chromatids separate
Cytokinesis begins
Meiosis II – Telophase II
The chromosomes may decondense (depends on species)
Cytokinesis reaches
completion, creating four haploid daughter cells
https://www.youtube.com/watch?v=D1_-mQS_FZ0
Answer these questions… How does the number of daughter cells produced
from mitosis and meiosis differ? When mitosis is complete, there are two daughter cells. When
meiosis is complete, there are four. How does the ploidy of the daughter cells produced
from mitosis and meiosis differ? Mitosis produces diploid (2n) cells. Meiosis produces haploid (n)
cells. Do the daughter cells produced from mitosis contain
identical genetic complements? Yes, the purpose of mitosis is to produce two identical cells
Do any of the daughter cells produced from meiosis contain identical genetic complements? No, the genetic information swapped between homologous
chromosomes during crossing over insures that each daughter cell produced during meiosis will be unique
Answer these questions… When do the homologous chromosomes
separate during mitosis? Never, they are never joined during mitosis (no tetrads are
formed) When do the homologous chromosomes
separate during meiosis? Homologs separate during Anaphase I, when the tetrads
break When do sister chromatids separate during
mitosis? Sister chromatids separate during Anaphase.
When do sister chromatids separate during meiosis? Sister chromatids separate during Anaphase II.
The Consequences of Meiotic Mistakes
Nondisjunctions occur when homologous chromosomes fail to separate at meiosis I or when chromatids fail to separate at meiosis II.
The Consequences of Meiotic Mistakes Nondisjunctions occur when homologous
chromosomes fail to separate at meiosis I or when chromatids fail to separate at meiosis II Fertilization can result in embryos that are 2n + 1 (a
"trisomy") Abnormal copy numbers of one or more chromosomes is
usually, but not always, fatal (Example: Down syndrome)
Polyploidy can occur when whole sets of chromosomes fail to separate at meiosis I or II The resulting 2n gametes, if fertilized by normal sperm,
create 3n zygotes (triploid) Organisms with an odd number of chromosome sets
cannot produce viable gametes (Example: seedless fruits)
Answer these questions…
How do mutations drive evolution? Mutations change traits of and organism if
change helps the organism survive the greater the chance of that organism living long enough to reproduce trait gets passed down through generations
How does sexual reproduction drive evolution? Sexual reproduction may exist because it provides
genetic variability that reduces susceptibility of a population to pathogen attack
Evolution All of the similarities
and dissimilarities among groups of organisms that are the result of the branching process creating the great tree of life, were viewed by early 19th century philosophers and scientists as a consequence of omnipotent design.
Evolution
In 1859, Charles Darwin published his famous On the Origin of Species
Patterns in the distribution and similarity of organisms had an important influence of Darwin's thinking
Darwin’s Theory of Evolution
Species (populations of interbreeding organisms) change over time and space. The representatives of species living today differ from those that lived in the recent past, and populations in different geographic regions today differ slightly in form or behavior. These differences extend into the fossil record, which provides ample support for this claim.
Darwin’s Theory of Evolution All organisms share common ancestors with
other organisms. Over time, populations may divide into different species, which share a common ancestral population. Far enough back in time, any pair of organisms shares a common ancestor. For example, humans shared a common ancestor with chimpanzees about eight million years ago, with whales about 60 million years ago, and with kangaroos over 100 million years ago. Shared ancestry explains the similarities of organisms that are classified together: their similarities reflect the inheritance of traits from a common ancestor.
Darwin’s Theory of Evolution
Evolutionary change is gradual and slow in Darwin’s view. This claim was supported by the long episodes of gradual change in organisms in the fossil record and the fact that no naturalist had observed the sudden appearance of a new species in Darwin’s time.
Since then, biologists and paleontologists have
documented a broad spectrum of slow to rapid rates of evolutionary change within lineages.
The primary mechanism of change over time is natural selection
The Process of Natural Selection Variation
Organisms (within populations) exhibit individual variation in appearance and behavior Body size, hair color, facial markings, voice properties
etc. Some traits show little to no variation among
individuals Number of eyes in vertebrates
Inheritance Some traits are consistently passed on from
parent to offspring Other traits are strongly influenced by
environmental conditions and show weak heritability
The Process of Natural Selection
High rate of population growth Most populations have more offspring each year
than local resources can support leading to a struggle for resources.
Each generation experiences substantial mortality.
Differential survival and reproduction Individuals possessing traits well suited for the
struggle for local resources will contribute more offspring to the next generation.
Final Thoughts on Evolution
In order for natural selection to operate on a trait, the trait must possess heritable variation and must confer an advantage in the competition for resources. If one of these requirements does not occur, then the trait does not experience natural selection
“…as natural selection acts by competition for resources, it adapts the inhabitants of each country only in relation to the degree of perfection of their associates” (Charles Darwin, On the Origin of Species, 1859).
Final Thoughts on Evolution
Variations arise by mutation Mutations arise by chance and without
foresight for the potential advantage or disadvantage of the mutation.
In other words, variations do not arise
because they are needed.