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Chapter 11- Genetics Meiosis Principles of genetics require:

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1 Chapter 11- Genetics Meiosis Principles of genetics require:
-each organism inherits a single copy of every gene from each parent -during gamete formation, two sets of genes must separate, so each gamete contains just one set of genes

2 Chapter 11- Genetics Chromosome Number Fruit fly example:
8 chromosomes total 4 came from male parent 4 came from female parent Two sets are homologous

3 Chapter 11- Genetics Homologous chromosomes- paired chromosomes having genes for the same trait located at the same place on the chromosome Diploid- having two of each kind of chromosome (2n); normal body cells Diploid cells have two complete sets of chromosomes/two complete set of genes

4 Chapter 11- Genetics Gametes of sexually reproducing organisms have only a single set of chromosomes Haploid- having one of each kind of chromosome (n) Fruit fly gametes would be n = 4


6 Chapter 11- Genetics Phases of Meiosis
How haploid cells (gametes) produced from diploid cells (somatic cells) Meiosis- cell division that results in a gamete containing half the number (n) of chromosomes of its parent (2n) Involves two rounds of division: -Meiosis I -Meiosis II


8 Chapter 11- Genetics Phases of Meiosis Meiosis I Similar to mitosis
Four phases the same: prophase I, metaphase I, anaphase I, and telophase I

9 Chapter 11- Genetics Differences- Prophase I:
Pairing of homologous chromosomes to form tetrad (4 chromatids) Crossing over- exchange of genetic material between non-sister chromatids of homologous chromosomes Shuffles genes like a deck of cards


11 Chapter 11- Genetics Meiosis II Second round of division (reduction)
NO chromosome replication Phases just like meiosis I: Prophase II, metaphase II, anaphase II, telophase II At end of division cycle, each cell contains haploid # of chromosomes (n)

12 Chapter 11- Genetics Gamete Formation
In male animals, gametes produced through meiosis called sperm In some plants, pollen contains sperm cells In female animals, only one cell produced is made into a gamete Cytokinesis is uneven: produces one egg and 3 polar bodies

13 Chapter 11- Genetics Comparing Mitosis & Meiosis Mitosis Meiosis
2 identical diploid cells 4 genetically different haploid cells Multicellular organisms: growth & repair Formation of gametes Asexual reproduction Sexual reproduction

14 Chapter 11- Genetics The Work of Gregor Mendel
Genetics- scientific study of heredity Gregor Mendel- Austrian monk who experimented using pea plants Determined rules of inheritance Fertilization- joining of male & female gametes to form single cell

15 Chapter 11- Genetics Pea plants normally self-pollinating
Mendel used stock of true-breeding plants (produced offspring identical to themselves) Cross-pollinated pea plants with different traits to study resulting offspring

16 Chapter 11- Genetics Genes & Dominance
Studied 7 different pea plant traits Trait- a characteristic that can be passed on to offspring For each trait, studied a contrasting character: Ex. Seed color- green or yellow


18 Chapter 11- Genetics Original pair of plants = P generation
Offspring = F1 generation Hybrid- offspring of parents that have different forms of a trait Results? Every offspring had a character of only one of the parents


20 Chapter 11- Genetics Two conclusions:
Inheritance determined by factors passed from one generation to the next Gene- the functional units of inheritance Each trait controlled by one gene with two contrasting forms Alleles- alternate forms of a gene


22 Chapter 11- Genetics Second conclusion called the principle of dominance: Some alleles are dominant and others are recessive Dominant- an allele of a gene that is always expressed (T) Recessive- an allele of a gene that is expressed only when the dominant allele is not present (t)

23 Chapter 11- Genetics Segregation
What had happened to recessive alleles (traits) in plants? Mendel allowed F1 plants to self-pollinate Produced F2 generation which showed recessive trait in 1 out of 4 plants (3:1 ratio)


25 Chapter 11- Genetics Dominant allele masked recessive trait in F1 generation Reappearance in F2 generation showed that at some point alleles became separated Mendel suggested that they segregated from one another during gamete formation


27 Chapter 11- Genetics Law of Segregation- states that every individual has two alleles of every gene; when gametes are produced, each gamete receives one of these alleles

28 Chapter 11- Genetics Probability & Punnett Squares
Every time Mendel repeated a particular cross, he obtained similar results ¾ plants showed dominant trait; ¼ showed recessive trait Mendel realized that probability could be used to explain results

29 Chapter 11- Genetics

30 Chapter 11- Genetics

31 Chapter 11- Genetics Genetics & Probability
Probability- the likelihood that a particular event will occur Two possible outcomes for a coin flip: heads or tails Probability of each occurring is equal Chance of heads is 1 in 2, or 50%

32 Chapter 11- Genetics Chance of heads coming up three in a row?
Each flip is an independent event: Probability of three heads in a row- ½ x ½ x ½ = 1/8 Way alleles segregate is random, like a coin flip Principles of probability can be used to predict outcomes of genetic crosses

33 Chapter 11- Genetics Punnett Squares
Gene combinations from a given cross can be determined using a Punnett square Letters in Punnett square represent alleles Gametes from each parent are shown along one side and the top of the square Used to predict and compare genetic variations resulting from a cross


35 Chapter 11- Genetics Homozygous- if an organisms two alleles for a trait are the same Can be homozygous dominant (TT) or homozygous recessive (tt) Heterozygous- when the two alleles for the same trait are different (Tt) Homozygous = true-breeding for trait Heterozygous = hybrid for trait

36 Chapter 11- Genetics Phenotype- the physical expression of the genes; how the trait looks Ex.- Tall; short; yellow seeds; green seeds Genotype- allele combination that an organism contains TT; Tt; tt

37 Chapter 11- Genetics Probability & Segregation
Probability predicts that if the alleles segregate, then a 3:1 phenotypic ratio should be seen Each cross showed a 3:1 ratio Mendel’s law of segregation proven correct

38 Chapter 11- Genetics Since probabilities predict averages:
the larger the number of individuals, the closer the data comes to the probability

39 Chapter 11- Genetics Exploring Mendelian Genetics
Mendel further investigated: Alleles segregate- but do alleles package together? or are they independent of one another?

40 Chapter 11- Genetics Independent Assortment
Mendel followed two different genes through two generations Crossed purebred plants for seed color and seed shape Round, yellow seeds (RRYY) with Wrinkled, green seeds (rryy)

41 Chapter 11- Genetics Only provided hybrid plants needed for next cross
All F1 plants had genotype RrYy F2 generation showed 209 plants that had phenotypes not found in the parents So… Alleles for different traits segregate independently of one another


43 Chapter 11- Genetics Summary of Mendel’s Principles
Inheritance is determined by individual units called genes. Genes passed from parents to offspring. When two or more forms (alleles) of a gene exist, some forms may be dominant, others recessive.

44 Chapter 11- Genetics In most sexually reproducing organisms, each adult has two copies of each gene – one from each parent. These copies are segregated from one another during meiosis (gamete formation) Alleles for different genes usually segregate independently of one another

45 Chapter 11- Genetics Complex patterns of Inheritance
Important exceptions to Mendel’s principles Some alleles are neither dominant nor recessive Many traits controlled by multiple alleles, or multiple genes

46 Chapter 11- Genetics Incomplete dominance
Appearance of a third phenotype Due to one allele being not completely dominant over another 3rd phenotype somewhere in between the two homozygous phenotypes


48 Chapter 11- Genetics Codominance Expression of both alleles
(ex. White bull crossed with a red cow yields a roan calf) Both alleles contribute to the phenotype

49 Chapter 11- Genetics Multiple phenotypes from multiple alleles
More common for multiple alleles to control a trait in a population Only two alleles of a gene can exist within the cell Multiple alleles for a single gene can be found within a population Examples: Blood type- A, B, o Rabbit coat color- C, cch, ch, c


51 Chapter 11- Genetics Polygenic Inheritance
Traits that are determined through expression of two or more genes Polygenic traits show wide range of phenotypes Ex- eye color, skin color, height



54 Chapter 11- Genetics Genetics & the Environment
Characteristics of any organism not solely determined by genes Determined by interaction between genes and environment Genes provide plan, how plan goes also depends on environment

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