7th grade life science major concepts/ skills
TRANSCRIPT
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GENES AND HEREDITY
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The Standards
What we are asked
to teach our students
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7th Grade Life ScienceMajor Concepts/ Skills
• Diversity of living • Dichotomous key/classify (6 Kingdoms)• Structure and function of cells • Tissues, organs, and organ systems • Purpose of major human body organ systems• Heredity, genes, and successive generations• Ecosystems • Cycling of matter and energy • Biological evolution • Natural selection and fossil record
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S7L3. Students will recognize how biological traits are passed on to successive generations.
a. Explain the role of genes and chromosomes in the process of inheriting a specific trait.
b. Compare and contrast that organisms reproduce asexually and sexually (bacteria, protists, fungi, plants & animals).
c. Recognize that selective breeding can produce plants or animals with desired traits.
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Translating the Standards
What are we trying to help the students understand?
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a. Explain the role of genes and chromosomes
in the process of inheriting a specific trait
• Students need to know what genes and chromosomes are– What is their structure?– How genes code for the proteins that result in
interaction with the environment
• Students need to know the difference between the genotype and phenotype of an organism
• Students need to know the basics of Mendelian genetics and inheritance
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b. Compare and contrast that organisms reproduce asexually and sexually (bacteria, protists, fungi, plants & animals).
• Students need to know that bacteria reproduce asexually by binary fission
• Students need to know that mitosis is a type of nuclear and cell division that is used both in growth and asexual reproduction
• Students need to know that meiosis is a type of nuclear and cell division that is used to make the sperm and eggs used in sexual reproduction
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c. Recognize that selective breeding can produce plants or animals with desired traits.
• For students to understand selective breeding (artificial selection) they need to recognize that:– Genotype controls phenotype and that a
desirable trait is a phenotype– A population of organisms usually has large
genetic variation and reflected in a variety of forms of a trait
– The only difference between artificial selection and natural selection is the source of the selective pressure
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Major Topics inGenes and Heredity
• The Inheritance of Traits
• Mendelian Genetics: When the Role of Genes Is Clear
• Quantitative Genetics: When Genes and Environment Interact
• Genes, Environment, and the Individual
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The Inheritance of Traits
• Most children are similar to their parents
• Children tend to be similar to siblings
• Each child is a combination of parental traits
• The combination of paternal traits and maternal traits is unique for each individual child
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The human life cycle
• gametes (a male sperm cell + a female egg cell) fuse during fertilization to form a single celled zygote, or embryo
• the embryo grows by cell division in mitosis
• the embryo grows into a child• the child matures into an adult
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Genes
• Most genes are segments of DNA that carry information about how to make proteins
– Structural proteins – for things like hair
– Functional proteins – for things like breaking down lactose
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Genes
• All cells have the same genes• Only certain genes are active in a single
cell– Heart cells and eye cells have genes for
the protein rhodopsin, which helps to detect light
– This is only produced in eye cells, not heart cells
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Genes and Chromosomes
• DNA is sort of like an instruction manual that shows how to build and maintain a living organism…
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Genes Are on Chromosomes• The genes are located on the
chromosomes
• The number of chromosomes depends on the organism
– Bacteria – one circular chromosome
– Humans – 23 homologous pairs of linear chromosomes
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Genes Are on Chromosomes• Each of the 23 pairs of chromosomes is a
homologous pair that carry the same gene
• For each homologous pair, one came from mom and the other from dad
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Gene Variation Is Caused by Mutation
• Genes on a homologous pair are the same, but the exact information may not be the same
• Sometimes errors or mutations in gene copies can cause somewhat different proteins to be produced
• Different gene versions are called alleles
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Diversity in Offspring
• The combination from the parents creates the individual traits of each child
• Environment also plays a role, but differing alleles from parents are the primary reason that non-twin siblings are not identical
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Diversity in Offspring
Non-twin siblings:
• The combination each individual receives depended on the gametes that were part of the fertilization event
• Remember that each gamete has 1 copy of each homologous pair
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Segregation
• When a gamete is formed, the homologous pairs are separated and segregated into separate gametes (this is called the law of segregation)
• This results in gametes with only 23 chromosomes– 1 of each homologous pair
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Independent Assortment
• Due to independent assortment, parents contribute a unique subset of alleles to each of their non-identical twin offspring
• Since each gamete is produced independently, the combination of genes is unique
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Diversity in Offspring
• That means a unique egg will be fertilized by a unique sperm to produce a unique child
• For each gene, there is a 50% chance of having the same allele as a sibling
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Diversity in Offspring
• There are 223 combinations for the way the homologous chromosomes could line up and separate
• This is more than 8 million combinations
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Crossing Over
• In addition, crossing over in meiosis can increase diversity
• The chromosomes trade information, creating new combinations of information
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Random Fertilization
• Gametes combine randomly—without regard to the alleles they carry in a process called random fertilization
• You are one out of 64 trillion genetically different children that your parents could produce
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Diversity in Offspring
• Mutation, independent assortment, crossing over, and random fertilization result in unique combinations of alleles
• These processes produce the diversity of individuals found in humans and all other sexually reproducing biological populations
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Twins
• Fraternal (non-identical)
– dizygotic: two separate fertilized eggs
– not genetically the same
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Twins
• Identical
– monozygotic: one single fertilized egg that separates
– genetically the same
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Mendelian Genetics:When the Role of
Genes Is Clear
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Gregor Mendel
• Determined how traits were inherited
• Used pea plants and analyzed traits of parents and offspring
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Mendelian Genetics
• Mendelian genetics – the pattern of inheritance described by Mendel – for single genes with distinct alleles
• Sometimes inheritance is not so straightforward
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Genotype
• Genotype – combination of alleles
– homozygous: two of the same allele
– heterozygous: two different alleles
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Phenotype
• Phenotype
– the physical outcome of the genotype
– depends on nature of alleles
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Mendelian Genetics
• Dominant – can mask a recessive allele
• Recessive – can be masked by a dominant allele
• Incomplete dominance – alleles produce an intermediate phenotype
• Codominance – both alleles are fully expressed
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Mendelian Genetics
Dominant alleles – capital letter
• For example: T for tall
Recessive alleles – lower case letter
• For example: t for short
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Genetic Diseases in Humans
• Most alleles do not cause diseases in humans
• There are some diseases that are genetic:
– Recessive, such as cystic fibrosis
– Dominant, such as Huntington’s Disease
– Codominant, such as sickle-cell anemia
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Genetic Diseases:Cystic Fibrosis
• Affects 1 in 2500 individuals in European populations
• Recessive condition: individuals have 2 copies of cystic fibrosis allele
• Carriers – have one cystic fibrosis allele but do not have cystic fibrosis; can pass along allele to children
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Genetic Diseases:Cystic Fibrosis
• Produces nonfunctioning proteins
• Normal protein transports chloride ion in and out of cells in lungs
• Result – thick mucus layer that is difficult from lungs and interferes with absorption of nutrients in intestines
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Huntington’s Disease
• Dominant condition
• Fatal condition
• Only one Huntington’s allele needed
• Produces abnormal protein that clumps up in cell nuclei – especially nerve cells in the brain
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Sickle-Cell Anemia
• Codominant – both alleles are expressed
• One allele codes for normal hemoglobin and the other codes for sickle-cell hemoglobin
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Sickle-Cell Anemia
• If you have two normal hemoglobin alleles, you do not have the disease
• If you have two sickle-cell hemoglobin alleles, you have sickle-cell disease
• If you have one of each, you are a carrier
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Punnett Squares
• Punnett squares are used to predict offspring phenotypes
• Uses possible gametes from parents to predict possible offspring
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Punnett Squares: Single Gene• A parent who is heterozygous for a trait
– Aa can produce two possible gametes
A or a
• A parent who is homozygous for a trait
– AA can only produce gametes with A
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Punnett Squares• The possible gametes are listed along the
top and side of the square
• The predicted offspring genotypes are filled in the center boxes of the square
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Punnett Squares
• The offspring can be homozygous or heterozygous
• It all depends on the parents and the possible gametes
• Punnet squares can be used to predict possibilities of inheriting genetic diseases
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Punnett Squares
• This is a probability for each individual offspring
• If there is a 25% chance an offspring will have cystic fibrosis – this means that – for every fertilization event, there is a 25% chance of cystic fibrosis
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Punnett Squares: Multiple Genes• You can also use Punnett squares to
predict the offspring with multiple genes
• It is more significantly more difficult as the number of genes being studied increases
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Quantitative Genetics
• The environment plays a role – traits such as height, weight, musical ability, susceptibility to cancer, and intelligence
• Quantitative traits show continuous variation; we can see a large range of phenotypes in the population
• The amount of variation in a population is called variance
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Why Traits Are Quantitative
• Polygenic traits – those traits influence by more than one gene
• Eye color is a polygenic trait
• There are two genes: pigment and distribution
• This produces a range of eye colors
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Why Traits Are Quantitative
• Environment can affect phenotypes
• Identical twins with the same genotypes may not have exactly the same appearance…
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Why Traits Are Quantitative
• Skin color is affected by both genes and environment…
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Why Traits Are Quantitative
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Using Heritabilityto Analyze Inheritance
• Inheritance patterns for these quantitative traits are difficult to understand
• Researchers use plants and domestic animals to study heritability – a measure of the relative importance of genes in determining variation in quantitative traits among individuals
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Using Heritabilityto Analyze Inheritance
• Artificial selection:
– controlling the reproduction of organisms to achieve desired offspring
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Calculating Heritabilityin Human Populations
• We can’t use artificial selection for humans
• So we look at correlations
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Correlations betweenParents and Children
• Inject parent and offspring in a bird population
• Look for correlation between parents and offspring in ability to produce anti-tetanus proteins
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Correlations betweenParents and Children
• For human IQ, the correlation between parents and offspring is 0.42
• There is also an effect of society and environment on IQ
• Nature versus nurture debate – “born that way” or because they were “raised that way”
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Correlation between Twins
• Twin studies allow scientists to test the effects of environment
• The DNA is identical in identical twins but the environment may be different
• Compare monozygotic (identical) twins to dizygotic (fraternal) twins
• Study twins raised together and study identical twins raised apart
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Genes, Environment, and the Individual
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The Use and Misuse of Heritability
• Calculated heritability values are unique to a particular environment
• Therefore, we must be cautious when using heritability to measure the general importance of genes to the development of a trait
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The Use and Misuse of Heritability
• The environment may cause large differences among individuals, even if a trait has high heritability
• Highly heritable traits can respond to environmental change
• Traits can be both highly heritable and strongly influenced by the environment
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The Use and Misuse of Heritability
• Knowing the heritability of a trait does not tell us why two individuals differ for that trait
• Our current understanding of the relationship between genes and complex traits does not allow us to predict the phenotype of a particular offspring from the phenotype of its parents